U.S. President Donald Trump has agreed with British Prime Minister Theresa May that Moscow must provide "unambiguous answers" about how a Russian-developed chemical agent was used against a former Russian spy in Britain, the White House said on March 13.

"The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms," the White House said in a brief statement following a telephone conversation between Trump and May.

May's spokesman said in a separate statement that Trump "said the U.S. was with the U.K. all the way."

Earlier in the day, Trump told reporters Washington will condemn "Russia or whoever" was behind the exposure of Sergei Skripal and his daughter, Yulia, to a toxic nerve agent in the English city of Salisbury on March 4 once it has the facts.

"It sounds to me that it would be Russia based on all the evidence they have," Trump said, referring to British authorities. "As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be."

Tension is rising over what Britain says was a deliberate attack using a highly toxic, military-grade nerve agent from a group of such poisons known as Novichok, which was developed in the Soviet Union.

A day after May said it was "highly likely" that Russia was behind the attack, Britain's ambassador to the Organization for the Prohibition of Chemical Weapons said that Russia had failed to declare its full stockpile of nerve agents to the global oversight body.

Peter Wilson told reporters that Russia, which President Vladimir Putin said in 2017 had destroyed all its chemical weapons under OPCW supervision, has in fact "failed for many years" to fully disclose its chemical weapons program.

Wilson repeated assertions by the British government that Russia was "implicated" in the attack and demanded that Moscow now declare its undisclosed program.

Trump and Wilson both spoke after Russian Foreign Minister Sergei Lavrov denied that Moscow was behind the poisoning of Skripal and his daughter and rejected London’s demand for an explanation of the incident.

"Russia is not guilty, " Lavrov told a news conference in Moscow. He made clear that Moscow would not meet Britain's demand that it explain, by the end of the day, how the nerve agent got to Salisbury.

But British Home Secretary Amber Rudd said the demand and the deadline were still in place.

"The Russians have started responding. The prime minister has been very clear that they have until midnight tonight to satisfy her requests," said Rudd, Britain's top police official. "Until then, we will wait and see what they have put forward."

Meanwhile, British broadcasting regulator Ofcom has warned it could review the license of Kremlin-backed broadcaster RT.

Ofcom said it had written to ANO TV Novosti, which holds RT's British broadcast licenses, saying that if Moscow is found to be behind the attack, "we would consider this relevant to our ongoing duty to be satisfied that RT is fit and proper."

In response to Ofcom's statement, Moscow threatened on March 13 to bar all British media from working in Russia if British authorities ban RT.

"No British media will work in Russia if they close down RT," Russian Foreign Ministry spokeswoman Maria Zakharova said.

Also on March 13, the chief of the OPCW said it is "extremely worrying that chemical agents are still being used to harm people."

"Those found responsible for this use must be held accountable for their actions," OPCW Director-General Ahmet Uzumcu said in a speech to the group's executive council.

Uzumcu said that British Foreign Secretary Boris Johnson called him on March 12 to inform him of the results of investigations into the attack on Skripal, 66, and his 33-year-old daughter Yulia.

The pair were found unconscious on a bench outside a shopping mall in Salisbury on March 4 and taken to the hospital, where they remain in critical but stable condition. A police officer involved in the response is in very serious condition but is able to speak.

The incident has caused a spike in tensions between Britain and Russia, whose ties have been strained since the painful radioactive-isotope poisoning death of Kremlin critic and former Russian security agent Aleksandr Litvinenko in London in 2006.

A British inquiry concluded that the Russian government was behind Litvinenko's death and that Putin "probably approved" the killing. Russia has denied involvement.

The poisoning in Salisbury has also prompted angry reactions from the United States and other Western countries, adding to persistent tensions with Moscow over Russia's aggression in Ukraine, its role in the Syria war, and other issues.

Speaking in Parliament on March 12, May said it is "now clear that...Skripal and his daughter were poisoned with a military-grade nerve agent of a type developed by Russia.

“The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal.”

Lavrov, in the most extensive comments from a senior Russian official since the poisoning, dismissed the accusations against Russia as "nonsense," adding: "We have nothing to do with it."

He suggested that Britain had not followed proper procedures for making accusations about alleged chemical attacks and had refused to provide Moscow with materials related to the case.

"Russia is ready to cooperate in accordance with the [Chemical Weapons Convention] if the United Kingdom...condescends to fulfill its international legal obligations under the same document," he said.

On March 12, U.S. Secretary of State Rex Tillerson -- whose dismissal was announced by Trump on March 13 -- said the United States is "outraged" over the incident and warned that Moscow "certainly will" face an allied response over the matter.

"We have full confidence in the U.K.'s investigation and its assessment that Russia was likely responsible for the nerve-agent attack that took place in Salisbury last week," Tillerson said.

"There is never a justification for this type of attack – the attempted murder of a private citizen on the soil of a sovereign nation – and we are outraged that Russia appears to have again engaged in such behavior," Tillerson said.

"From Ukraine to Syria – and now the U.K. – Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens," he said.

"We agree that those responsible -- both those who committed the crime and those who ordered it -- must face appropriately serious consequences," Tillerson said. "We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses."

Tillerson's vow of allied action in response to the attack was echoed by other British allies.

At a briefing on March 12, White House spokeswoman Sarah Sanders said that the "use of a highly lethal nerve agent against U.K. citizens on U.K. soil is an outrage," but did not mention Russia.

"The attack was reckless, indiscriminate, and irresponsible. We offer the fullest condemnation...and our support to the U.K. government," Sanders said. "We stand by our closest ally and the special relationship that we have."

NATO Secretary-General Jens Stoltenberg said "the U.K. is a highly valued ally, and this incident is of great concern to NATO.... The use of any nerve agent is horrendous and completely unacceptable."

In a phone call with May, French President Emmanuel Macron "offered his solidarity with the U.K.," Downing Street said.

"They discussed the wide pattern of aggressive Russian behavior and agreed that it would be important to continue to act in concert with allies to address it," it said.

Skripal, a retired Russian military intelligence colonel, was convicted by a Moscow military court in 2006 of "high treason" for passing secrets to Britain's Secret Intelligence Service.

He was one of four Russian prisoners released in 2010 in exchange for 10 Russian sleeper agents detained in the United States in one of the biggest spy scandals since the Cold War.

May said there were only two possible reasons for the detection of the Russian-made nerve agent: that it was used in "a direct action by the Russian state" or because of Russia "losing control" of its stocks of the deadly chemical.

"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom," May said, calling the attack a "reckless and despicable act."

"We will not tolerate such a brazen attempt to murder innocent civilians on our soil," she added.

After May pointed the finger at Russia, the British Foreign Office summoned the Russian ambassador and British authorities issued the demand for an explanation by the end of the day on March 13.

Russian Foreign Ministry spokeswoman Maria Zakharova called May's remarks a "circus show" and the ministry said on March 13 that it had summoned the British ambassador.

In a separate statement on March 12, the Russian Foreign Ministry -- without specifically denying Russian involvement -- suggested that the accusations were aimed to create a pretext from Britain to boycott the 2018 soccer World Cup in Russia this summer.

It said it had warned repeatedly that Western media would roll out "a full-scale...campaign with the aim of discrediting Russia and undermining trust in it as the host" of the June-July tournament, in which England is scheduled to compete.

Before making his statement, Tillerson called his British counterpart, Foreign Secretary Boris Johnson, to discuss the poisoning.

Speaking to reporters during a flight back to Washington from Africa, Tillerson agreed with May's assessment that the attackers had used a nerve agent developed by Russia.

"It appears that it clearly came from Russia. Whether it came from Russia with the Russian government's knowledge is not known to me at this point," he said.

"This is a substance that is known to us and does not exist widely. It is only in the hands of a very, very limited number of parties. And I don't want to say anything further than that," he told reporters.

"You take a substance like was used, which is an extremely dangerous substance, into another country, into a public place, where you know many others are going to be exposed...it's almost beyond comprehension that a state, an organized state, would do something like that," Tillerson said.

Asked whether the apparent attack on a NATO member would trigger an allied response, Tillerson said: "It certainly will trigger a response. I'll leave it at that."

Tillerson told reporters that he's grown "extremely concerned" about Russia, noting that he spent most of his first year in office trying to solve problems and narrow differences with the Kremlin. He said after a year of trying, "We didn't get very far."

"Instead what we've seen is a pivot on their part to be more aggressive," Tillerson said. "And this is very, very concerning to me and others that there seems to be a certain unleashing of activity that we don't fully understand what the objective behind that is."

With reporting by AP, AFP, dpa, and Reuter