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

The meeting with 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 US 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 US 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 United States, 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 alliance, which partly promises US support for European allies against Russian aggression. Trump has worked to assure Europe he will continue to support the alliance but insists that member countries meet their financial obligations to ‘‘pay their fair share.’’

In a meeting 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 administration announced sanctions on two Russian officials and three dozen individuals and companies over Russian activities in Ukraine. Treasury Secretary Steven Mnuchin said the goal is to ‘‘maintain pressure on Russia to work toward a diplomatic solution.’’

Proponents of Senate-passed legislation to hit Russia with economic sanctions and limit the president’s authority to lift the penalties fear Trump may seek to dilute the bill and are urging the House to act quickly.

Senator John McCain of Arizona, called for the House to vote as soon as possible. McCain, who has been pushing for the United States to respond to Russia’s US election meddling, predicted the legislation would pass overwhelmingly, just as it did in the Senate last week.

‘‘We all know that the Russians tried to interfere in our elections,’’ McCain said. ‘‘Here we are six months later and we’ve done nothing.’’