KIEV, Ukraine — Ominous new action by Ukraine’s security forces on Monday, including a raid on an opposition party’s headquarters, appeared to diminish prospects for talks between the government and protest leaders, as Western officials grasped for a way to defuse the country’s intensifying political crisis.

In a sign of mounting alarm, Vice President Joseph R. Biden Jr. and President José Manuel Barroso of the European Commission each called Ukraine’s besieged president, Viktor F. Yanukovich, to warn him against unleashing force on a mass demonstration movement in its third week. Senior envoys, including the European foreign affairs chief, Catherine Ashton, and an assistant secretary of state, Victoria Nuland, were sent in an effort to ease tensions.

The renewed diplomatic maneuvering was intended to prevent a repeat of the bloodshed during a violent crackdown by the police on Nov. 30 and to contain the widening civil uprising, which has plunged this nation of 46 million into deep uncertainty. Caught in a tug of war between European and Russian interests, Ukraine faces a severe economic crisis and is in immediate need of an aid package that is unlikely to materialize while thousands of protesters and riot police officers are massed in the streets.

In a meeting with senior Kremlin officials in Moscow on Monday, the State Department said, Ms. Nuland “urged Russia to use its influence to press for peace, human dignity and a political solution” and emphasized Ukraine’s need for “a return to economic health with the support of the International Monetary Fund.”