WASHINGTON — As American intelligence agencies have detected new Russian tanks and artillery crossing the border into Ukraine in recent days, President Obama is coming under increasing pressure from both parties and more officials inside his own government to send arms to the country. But he remains unconvinced that they would help.

Democrats joined Republicans on the Senate Foreign Relations Committee on Tuesday in unanimously pressing the administration to send weapons to Kiev. Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, publicly urged Mr. Obama to consider such a move last week, joining Defense Secretary Ashton B. Carter and James R. Clapper Jr., the director of national intelligence.

But the president has signaled privately that despite all the pressure, he remains reluctant to send arms. In part, he has told aides and visitors that arming the Ukrainians would encourage the notion that they could actually defeat the far more powerful Russians, and so it would potentially draw a more forceful response from Moscow. He also wants to give a shaky cease-fire a chance to take hold, despite a reported 1,000 violations so far, and seems determined to stay aligned with European allies that oppose arms for Ukraine.

“If you’re playing on the military terrain in Ukraine, you’k. “It has a huge amount of military equipment and military force right on the border. Anything we did as countries in terms of military support for Ukraine is likely to be matched and then doubled and tripled and quadrupled by Russia.”

That argument seems to most closely channel the president’s, according to people familiar with the internal debate. Mr. Obama continues to pose questions indicating his doubts. “O.K., what happens if we send in equipment — do we have to send in trainers?” said one person paraphrasing the discussion on the condition of anonymity. “What if it ends up in the hands of thugs? What if Putin escalates?”

But while Mr. Obama’s national security adviser, Susan E. Rice, shares his skepticism, the president finds himself increasingly flanked inside and outside his government by others urging him to do more to help the Ukrainians defend themselves.

Last week General Dempsey told lawmakers that “I think we should absolutely consider providing lethal aid.” Madeleine K. Albright, a secretary of state under President Bill Clinton; Zbigniew Brzezinski, who was national security adviser to President Jimmy Carter; and Michael A. McFaul, who was Mr. Obama’s ambassador to Moscow, have all said the same.

Representative Eliot L. Engel of New York, the top Democrat on the House Foreign Affairs Committee, said last week that he was so “disappointed” in the administration for not using tools in past legislation authorizing more sanctions against Russia and arms for Ukraine that he was introducing a new bill to “dial up the pressure on Vladimir Putin.”

A hearing of the Senate Foreign Relations Committee on Tuesday made clear that the disappointment was becoming widespread and bipartisan. “There’s no question there’s strong consensus on this committee, and in the United States Senate, that the United States needs to do more to help the Ukrainians defend themselves,” said Senator Benjamin L. Cardin, a Maryland Democrat.

The Obama administration has committed to sending Ukraine $118 million in nonlethal aid, like night-vision goggles and counter-mortar radar, but so far only about half has been delivered, officials told the panel. Brian P. McKeon, the principal deputy under secretary of defense, agreed that in some cases the aid has been “unacceptably slow” and said the administration was working to speed it up.

An announcement of more deliveries of that equipment may be made as early as Wednesday, and the administration also plans another $120 million of similar aid.

In resisting the pressure from advisers and fellow Democrats, Mr. Obama is adhering closely to European allies like Chancellor Angela Merkel of Germany, who has strongly opposed sending arms to Ukraine. A senior administration official confirmed Tuesday an Associated Press report that Mr. Obama told Ms. Merkel when she visited Washington last month that he would hold off sending weaponry during negotiations for a cease-fire. After that meeting, European, Russian and Ukrainian leaders meeting in Minsk, the capital of Belarus, crafted a cease-fire.

“We are all committed to making sure that we uphold the basic principles of sovereignty and territorial integrity that have been threatened by Russian aggression,” Mr. Obama said before a meeting on Monday with Donald Tusk, the president of the European Council. “We’ve been able to maintain strong unity with respect to sanctions.”

The senior administration official, who did not want to be identified describing Mr. Obama’s discussions with a foreign leader, said the president’s commitment to Ms. Merkel was temporary. “Going forward, we’ll have to make judgments based in part of our assessment of compliance with Minsk,” the official said. Other officials said no decision was likely anytime soon.

Even as the Minsk accord went into effect, pro-Russian separatists captured the city of Debaltseve. But in recent days, violence has fallen sharply, some heavy weapons have been withdrawn and a cautious optimism has emerged in Kiev and European capitals.

Still, American officials said the results of the cease-fire remain mixed. While Russia has denied arming and directing the separatists in eastern Ukraine, a top State Department official told the Senate panel on Tuesday that more military equipment has been sent across the border.

“Just in the last few days, we can confirm new transfers of Russian tanks, armored vehicles, heavy artillery and rocket equipment over the border to the separatists in eastern Ukraine,” Victoria Nuland, an assistant secretary of state, told the panel.

“So in the coming days, not weeks,” she added, “here’s what we need to see: a complete cease-fire in all parts of eastern Ukraine; full, unfettered access to the whole conflict zone; a pullback of all heavy weapons; and an end to uninspected convoys of cargo over the Ukrainian borders.”

Ms. Nuland said there was a “spirited debate” going on inside the administration over whether to send arms to Ukraine and that no decision has been made yet. But senators said delaying a decision was the same thing as deciding against sending arms.

Senator Jeanne Shaheen, Democrat of New Hampshire, said that it was “taking a very long time” to decide. Senator Robert Menendez of New Jersey, the ranking Democrat on the committee, said that by the time Mr. Obama decides “then it will be too late.” Senator Christopher S. Murphy, Democrat of Connecticut, was more sympathetic to the administration, admitting that it might not work but said “that’s a chance worth taking.”

Mr. McKeon said the administration was wrestling with the consequences of sending weapons to Ukraine.

“Does this raise the ante?” he asked. “And then what would Ukraine feel that the United States owes them in terms of additional assistance? So it’s trying to see down the field to the second, third and fourth move on this chessboard. That’s part of the conversation.”