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’re playing to Russia’s strength, because Russia is right next door,” Antony J. Blinken, the deputy secretary of state, told an audience in Berlin last week. “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.”