U.S. Defense Secretary Jim Mattis has accused Russia of seeking to “redraw international borders by force" and said that Washington is "actively reviewing" supplying Ukraine with lethal defensive weaponry.

Mattis, the first U.S. defense chief to visit Ukraine in a decade, also reiterated that the United States “won’t accept” Moscow's annexation of Ukraine's Crimea region.

Mattis made the comments in Kyiv on August 24, the 26th anniversary of Ukraine’s declaration of independence from the Soviet Union.

"Have no doubt," he said at a joint news conference with President Petro Poroshenko. "The United States stands with Ukraine."

"On the defensive lethal weapons, we are actively reviewing it," Mattis said, adding that he will inform Secretary of State Rex Tillerson and President Donald Trump about his position on the issue "in very specific terms."

"Defensive weapons are not provocative unless you are an aggressor, and clearly Ukraine is not an aggressor since it is their own territory where the fighting is happening," Mattis said, appearing to signal support for Kyiv’s request for defensive weaponry, including possibly powerful antitank missiles.

Mattis said sanctions against Moscow will remain in place until it stops supporting separatists in eastern Ukraine and returns Crimea, the Black Sea peninsula seized by Russia in March 2014.

He repeated Washington’s commitment to diplomatic efforts to resolve the conflict in Ukraine’s east, where fighting between Kyiv's forces and Russia-backed separatists has killed more than 10,000 people since April 2014.

The defense secretary also accused Russia of not abiding by the February 2015 Minsk agreement meant to put an end to the conflict.

"Despite Russia's denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe," Mattis said.

He also said that the United States is committed to “building the capacity” of Ukraine’s armed forces.

Russian officials and separatist leaders criticized the United States and Ukraine for discussing the provision of lethal weapons and rejected Mattis's assertion that doing so would not be "provocative."

The "ambition to acquire lethal weapons...is surely going to be perceived in Donbas as a very bad and threatening signal," said Russia's negotiator over the Ukraine conflict, Boris Gryzlov.

Gryzlov also rejected Mattis's suggestion that Ukraine is not an "aggressor."

"No one should forget the standoff in eastern Ukraine is an internal political conflict that the country's top echelons have already tried to curb by military force," he said.

Ukraine, the United States, and other Western countries strongly object to Russia's claim that the conflict is an internal one, saying that Russia fomented separatism before it broke out and has sent troops, weapons, and other support to the separatists.

Separatist leader Oleksandr Zakharchenko said, "As soon as Ukraine gets the weaponry, the Ukrainian Army will unleash

military actions automatically the next day."

Addressing a military parade attended by Mattis and several other Western defense chiefs, Poroshenko earlier said that "Ukraine is ready to give a tough military response to the aggressor if he tries to go on the offensive" -- a warning to Moscow and the separatists not to seek to take more territory in eastern Ukraine.

But he said that Ukraine's "priority" is a "peaceful, diplomatic, political, and law-based path to the return” of Crimea and separatist-held territory in the Donetsk and Luhansk regions.

"I have confidence in our allies," Poroshenko said. He thanked defense ministers and troops from Britain, Georgia, Estonia, Canada, Latvia, Lithuania, Moldova, Poland, Romania, and the United States for attending or marching in the parade.

Trump had sent a letter of congratulation to Poroshenko, saying that the United States will continue to support Ukraine's “sovereignty and integrity” and the country’s “aspirations of becoming a truly European nation,” according to the Ukrainian presidential website.

One incident of violence marred the otherwise peaceful independence celebration.

Police said at least three people were injured in an explosion caused by an unknown object in the center of the capital, an incident Ukrainian authorities described as “hooliganism.”

Kurt Volker, the U.S. special envoy for efforts to end the conflict in Ukraine, was also in the Ukrainian capital.

He told Current Time TV last month that the Trump administration is considering sending Kyiv weapons to help government forces defend themselves against the separatists.

Volker told the Russian-language network, which is run by RFE/RL in cooperation with Voice of America, that he did not think arming Ukraine with lethal defensive weapons would "provoke Russia to do more than they are already doing."

U.S. media reported on August 6 that the Pentagon had recommended sending a package of lethal defensive military aid to Ukraine worth about $50 million.

The weapons package would reportedly include Javelin shoulder-launched antitank missiles, which Kyiv has long sought to defend against the Russia-backed forces it has been fighting in its east for more than three years.

Ukraine’s Independence Day celebrations come as the Kyiv government and separatists committed to a cease-fire before the start of the September 1 return to school for children.

Martin Sajdik, the envoy for the Organization for Security and Cooperation in Europe (OSCE) on the Ukraine crisis, said on August 23 that an “indefinite” cease-fire would commence at midnight on August 25.

Several truce deals announced as part of the Minsk agreement have failed to hold.

In a statement, the chief monitor of the OSCE Special Monitoring Mission to Ukraine (SMM), Ertugrul Apakan, called the truce "an encouraging joint political signal from all signatories” of the Minsk agreement.

The deal set out steps to end the war and resolve the status of the portion of the Donbas region held by Russia-backed separatists, but progress toward implementation has been very slow.

The latest cease-fire was agreed late on August 22 during a phone call between the leaders of Germany, France, Russia, and Ukraine -- the so-called “Normandy Four.”

In the call, Poroshenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, and French President Emmanuel Macron all voiced strong support for a lasting cease-fire to allow children in eastern Ukraine to attend school at the start of the new term, the Kremlin and Poroshenko's press service said.

The United States and the European Union have imposed sanctions on Moscow for its annexation of Crimea and its support for the separatists in eastern Ukraine.

With reporting by AP, Reuters, AFP, TASS, and Interfax