President Donald Trump met with his Ukrainian counterpart Tuesday amid intensifying questions over whether his administration will step in to protect partners in the face of Russian aggression.

The meeting with Ukrainian President Petro Poroshenko was originally described by the White House as a brief "drop-in." But the two presidents posed for photographs in the Oval Office and made brief remarks following Poroshenko's more extensive meetings with Vice President Mike Pence and the administration's top national security advisers.

With the Ukrainian leader sitting by his side, Trump said it was a "great honor" to meet Poroshenko and that "a lot of progress has been made" in the U.S. relationship with Ukraine.

There was no mention of Russia, nor did Trump respond to questions about the ongoing investigation over possible collusion by members of the Trump administration with Russia during the 2016 presidential election. Trump staunchly denies he had any contact with Russian officials during the campaign and has tweeted that the investigation is a "witch hunt" spearheaded by Democrats bitter over losing the election.

Trump has maintained that he hopes to establish better ties with Moscow, repairing ill will from the Obama era that resulted from Russia's 2014 annexation of Ukraine's Crimea region and its widely condemned support of Syria's President Bashar Assad, despite his attacks against civilians. In April, following a suspected chemical attack against civilians in northern Syria by government forces, Trump said U.S. relations with Russia "may be at all-time low."

But less than a month later, the president hosted top Russian diplomats in a closed-door Oval Office meeting that suggested relations weren't so bad after all. Photos later emerged showing Trump laughing with Russian Foreign Minister Sergey Lavrov and Russia's envoy to the U.S., Sergey Kislyak. The White House press corps was not allowed access to that meeting.

Trump has also raised concerns among NATO allies about whether he would turn his back on the military alliance, which partly promises U.S. support for European allies against Russian aggression. Trump has worked to assure European leaders that he will continue to support the alliance but insists that member countries meet their financial obligations to "pay their fair share."

Poroshenko, speaking softly, said Tuesday he hopes the two countries can engage in "effective collaboration."

In his meeting later with Defense Secretary Jim Mattis, Poroshenko said, "I think that we can expand our cooperation because we fight not only for our territorial integrity and our independence, not only for our sovereignty, we are fighting for freedom, we are fighting for democracy."

The White House meeting began shortly after the Trump administration announced it has imposed sanctions on two Russian officials and three dozen other individuals and companies over Russian activities in Ukraine.

Treasury Secretary Steven Mnuchin said the new penalties are designed to "maintain pressure on Russia to work toward a diplomatic solution."

In all, Tuesday's action targets 38 individuals and firms. Any assets they have in the U.S. are now blocked. Americans are prohibited from doing business with them. The Russian officials affected are Moscow's envoy for overseas Russians and its chairman for humanitarian assistance in separatist-held, eastern Ukraine.

Lavrov, speaking in Moscow Tuesday following talks with his French counterpart Jean-Yves Le Drian, said the new restrictions "don't help to improve the climate." He added that he could "only voice regret about the Russo-phobic obsession of our American colleagues ... that has gone beyond any limits."

Tensions between the U.S. and Russia are high even as the White House considers scheduling a meeting with Russian President Vladimir Putin on the sidelines of next month's G20 meeting.

Russia threatened this week to target U.S. coalition planes after the U.S. shot down a Syrian fighter jet for the first time.

White House statements on Poroshenko's meetings with both Trump and Pence made no mention of Russia. However, White House spokesman Sean Spicer said Tuesday that the topic of Russia "obviously came up to discussion with the president today."

Spicer said the administration will continue to "advocate" for sanctions so long as Russia continues its aggression in eastern Ukraine.

Senate Armed Services Committee Chairman John McCain on Tuesday lambasted the administration's nominee to be the No. 2 official at the Pentagon, Boeing executive Patrick Shanahan, for initially sidestepping questions on whether he favors providing lethal weaponry to Ukraine. Shanahan told McCain he now supports it, but that did not seem to satisfy McCain, who said he was disappointed that the nominee did not initially have an opinion.

"Do not do that again, Mr. Shanahan, or I will not take your name up for a vote before this committee," said McCain.

The Pentagon said plans to provide lethal weapons are evaluated on a case-by-case basis.

"We've not provided lethal defensive weapons to Ukraine in the past, but we also don't rule out the option of doing so in the future," said Pentagon spokesman Jeff Davis.


Associated Press writers Robert Burns and Lolita Baldor contributed to this report.