Tom Vanden Brook, Jane Onyanga-Omara, and Gregory Korte

USA TODAY

WASHINGTON — Russia could have stopped its Syrian ally from conducting Tuesday's deadly poison gas attack that killed dozens of civilians, but did not, U.S. officials said Friday.

That assessment, from a senior military officer who spoke on condition of anonymity, went even further than Secretary of State Rex Tillerson's assessment that Russia was "complicit" or "simply incompetent" in enforcing a 2013 agreement that forced Syria to give up their chemical weapons.

And it raises the stakes for the U.S. involvement in the region, as the Trump administration tries to stanch the deepening humanitarian crisis in Syria and prevent President Bashar Assad from using chemical weapons. But those actions come at the risk of increased friction with Moscow, which has supported the Assad regime and condemned the U.S. airstrikes.

The U.S. airstrikes were carefully designed to avoid hitting Russian personnel at the Syrian air base, U.S. officials said Friday. But they said mere presence of as many as 100 Russian personnel at an air base used to launch a chemical attack raises questions about whether Russia knew about the attacks and failed to stop them. The defense officials spoke on condition of anonymity because they were not authorized to speak publicly about the operation.

White House press secretary Sean Spicer deflected questions about whether the U.S. was seeking to send a signal to Russia. "The actions that were taken were against the Assad regime, and I’m not going to say anything more than that," he said.

President Trump is "prepared to do more" to stop the civil war in Syria if Russia cannot convince President Bashar Assad to abide by ceasefire agreements, U.S. Ambassador to the United Nations Nikki Haley told the U.N. Security Council on Friday.

"We are prepared to do more, but we hope that will not be necessary. It is time for all civilized nations to stop the horrors that are taking place in Syria and demand a political solution," Haley said.

The U.S. missile attack, which was launched at 7:40 p.m. ET Thursday, was Trump's first major military action against another country and involved Syria, a nation that Trump had previously considered an ally of convenience in the fight against the Islamic State. But Trump's reluctance to involve U.S. forces in Syria began to melt Tuesday morning, when Trump was first presented with details of the chemical attack during his morning intelligence briefing. White House press secretary Sean Spicer gave a detailed timeline of Trump's deliberations Friday.

Trump asked for more information, and by 8 p.m. that night a deputies committee of the National Security Council deputies committee considered possible options for a strike.

During a full NSC meeting on Wednesday afternoon, Trump reviewed the options in detail and he gave the final approval for the attack at 4 p.m. Thursday after meeting in Florida with Defense Secretary Jim Mattis, National Security Adviser H.R. McMaster and Tillerson.

The U.S. strike, which Pentagon officials and Spicer called successful, led to a Syrian condemnation Friday morning. Assad's government called it a "blatant act of aggression."

Nine civilians were killed, including four children, when the projectiles hit the base and nearby villages, the official Syrian Arab News Agency reported. Several others were injured.

Assad's office called the strikes "shortsighted," “reckless” and “irresponsible.”

Timeline of Syria chemical attack

Two Pentagon officers sketched out the timeline leading up to Syria's April 4 attack on the rebel-held town of Khan Sheikhoun in Idlib province that killed 86 people, including 27 children.

The Syrian regime, feeling pressure from opposition forces and fearing the imminent loss of another key airfield, launched a series of chemical weapon attacks of increasing severity.

On March 25, Syrian aircraft dropped bombs carrying chlorine gas, a potentially lethal industrial agent. A subsequent attack, on March 30, appears to have used a nerve agent, the officers said.

The attack on April 4, almost certainly a chemical nerve agent, was the largest since 2013. That determination was made based on reports from Syria, although there has been no U.S. government validation. However, the Shayrat air base, where the planes carried out the attack and chemical weapons are believed to be stored, had been linked in the past to chemical weapons storage and military personnel trained to use them.

The attack occurred at about 6:50 a.m. local time. Within minutes, patients with symptoms of chemical exposure began streaming into local hospitals, the officers said.

Aerial imagery showed a small crater in a roadway consistent with a chemical weapon, the first of the two officers said. Destruction was minimal and there was staining around the crater's rim, indicating the presence of chemicals..

A drone circled the hospital, spying on the stream of ambulances and people entering. The drone left and returned. Soon after, a warplane attacked the hospital.

By April 5, Pentagon planners began developing military options for discussion by the National Security Council, top Pentagon officials and the president.

Four hours after Trump made the call to attack Thursday, 59 Tomahawk land-attack missiles launched from destroyers in the eastern Mediterranean Sea. Each one hit its mark, the officers said.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said six Syrian jets were destroyed but the air base's runway was intact. He said “the combat efficiency of the U.S. strike was very low" and 23 of the 59 missiles fired by the United States reached the base. "The place of the fall of the other missiles is unknown," Konashenkov said, according to Russia's TASS news agency.

The senior military officers flatly rejected that assessment, saying all 59 cruise missiles struck their targets, destroying an estimated 20 aircraft, hangars and other facilities at the airfield.

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Syria: U.S. now a partner of ISIS

The Syrian army said the missile strikes made the United States a “partner” of the Islamic State, Jabhat al-Nusra and other terrorist organizations.

Assad allies Russia and Iran also condemned the U.S. strikes, saying they violated international law.

"It is an act of aggression under a completely far-fetched pretext," Russian Foreign Minister Sergey Lavrov said Friday, according to TASS. "This is reminiscent of the situation in 2003, when the U.S. and the U.K., along with some of their allies, invaded Iraq without the consent of the U.N. Security Council and in violation of international law."

It's unclear how much the Kremlin knew about the U.S. strikes before they happened. The Kremlin said it received advance warning from the United States about the strikes.

“No contacts were made with Moscow, with President Putin," Tillerson said late Thursday. “There were no discussions or prior contacts, nor have there been any since the attack, with Moscow.”

But there were certainly military-to-military communications with Russian forces in the region, under a process known as "de-confliction." That hotline was set up during the Obama administration to ensure that U.S. and Russian forces don't accidentally clash when taking action against Islamic State targets.

Despite Thursday's action, Russia agreed to maintain that hotline, the two Defense officials said.

Konashenkov said Russia will help Syria strengthen its air defenses to help “protect the most sensitive Syrian infrastructure facilities," the Associated Press reported.

Iran said the U.S. action was "dangerous, destructive and violates the principles of international law.”

Iran’s Foreign Ministry spokesman, Bahram Ghasemi, said Iran condemned the strikes, “regardless of the perpetrators and the victims” of the chemical weapons attack and warned that the attack would “strengthen terrorists” and add to “the complexity of the situation in Syria and the region,” according to the semiofficial ISNA news agency.

Russia claimed the deaths Tuesday were caused by a Syrian strike on a terrorist chemical weapons facility, but the United States, other nations and human rights groups rejected that argument as baseless.

The 59 missiles, fired from the destroyers USS Porter and Ross in the eastern Mediterranean Sea, targeted the Shayrat Airfield in Homs province where Syria based warplanes used in the chemical attack, according to Navy Capt. Jeff Davis, a Pentagon spokesman. The missiles destroyed aircraft, hardened hangars, ammunition supply bunkers, air defense systems and radar at the base.

"As always, the U.S. took extraordinary measures to avoid civilian casualties and to comply with the Law of Armed Conflict. Every precaution was taken to execute this strike with minimal risk to personnel at the airfield," Davis said in a statement.

The Syrian army said the United States struck the air base without determining what happened or who was responsible for the chemical attack, which “sends wrong messages to the terrorist organizations that would embolden them further to use chemical weapons in the future every time they suffer heavy losses in the battlefield," SANA reported.

Russian presidential spokesman Dmitry Peskov said Vladimir Putin believed that Washington’s “total disregard for the use of chemical weapons by terrorists only exacerbates the situation significantly.”

"Putin also sees the strikes on Syria by the U.S. as an attempt to divert the attention of the international community from numerous civilians casualties in Iraq. Washington’s move impairs the Russian-U.S. relations, which are in a deplorable state, substantially," Peskov said, according to TASS.

Gregory Korte reported from Palm Beach, Fla. and Jane Onyanga-Omara reported from London.