Defense Secretary Jim Mattis, in Kiev for talks with Ukrainian President Petro Poroshenko, said Thursday the U.S. is actively considering providing defensive lethal weapons to assist Ukraine in fighting Russian separatists who control the eastern part of the country.

"On the defensive lethal weapons, we are actively reviewing it," Mattis said at a joint news conference with Poroshenko. "I will go back now having seen the current situation, and be able to inform the secretary of state and the president in very specific terms what I recommend for the direction ahead."

Poroshenko said his talks with Mattis included a discussion of "the enlargement of Ukraine's defense capabilities," which he said is not only about lethal weapons, but also improving his military's "electronic defenses."

"Talking over these issues requires silence," Poroshenko said through a translator. "Today I would abstain from commenting," but then added he is very satisfied with the support from its "strategic partner" the United States.

Mattis also said he owed President Trump some confidentiality on what he is advising, but rejected any suggestion that providing lethal weaponry would be a provocative step that would raise tensions with Russia.

"Defensive weapons are not provocative unless you are an aggressor," Mattis said "and clearly Ukraine is not an aggressor since it's their own territory where the fighting is happening."

The discussion occurred as congressional leaders are calling on President Trump to approve providing more defensive arms as a message to Russian President Vladimir Putin.

"Raising the cost of aggression may help to change Vladimir Putin's calculus, pressure Russia to fully comply with the Minsk agreements, and, ultimately, create more stable security conditions on the ground that are essential for peace," said Sen. John McCain, chairman of the Armed Services Committee, in a statement.

"As long as the status quo remains, Russia has no reason to change its behavior, and we should only expect more violence and more death," McCain said Wednesday.

Mattis was in Ukraine's capital Kiev as it celebrated its national day of independence, and issued a strong statement of support for the Poroshenko government.

"Have no doubt the U.S. stands with Ukraine in all things," Mattis said. "We do not, and we will not accept Russia's seizure of the Crimea. And despite Russia's denials we know that they are seeking to redraw international borders by force."

Mattis sharply criticized Moscow for failing to honor a series of agreements dating back to 1994, when it pledged in Budapest to honor Ukraine's sovereignty in return for Kiev giving up its vast arsenal of Soviet-era nuclear weapons, and continues with the cease-fire negotiated in Minsk in 2015.

"Russia put its reputation on the line when it accepted the Minsk cease-fire agreement," Mattis said. "Unfortunately, Russia is not adhering to the letter, much less the spirit of these international commitments."

The leaders of Russia, Ukraine, Germany, and France were all on a conference call Tuesday, according to the Kremlin, and discussed the latest efforts to secure a lasting cease-fire for eastern Ukraine in time for the beginning of the school year.