BEIRUT — The Syrian regime showed a new determination Wednesday to crush its opponents, vowing to “cleanse” a rebel-held district in the besieged central city of Homs after nearly four weeks of shelling.

Government troops massed outside the embattled neighborhood of Baba Amr, raising fears among activists of an imminent ground invasion that could endanger thousands of residents, as well as two trapped Western journalists, who have been under heavy bombardment.

A Spanish journalist who had been stuck in the area escaped Wednesday to Lebanon, the second foreign reporter to do so since a government rocket attack last week killed two of his colleagues and wounded two others.

The fate of the foreign journalists has drawn attention to Homs, which has emerged as a key battleground between government forces and those seeking to end the regime of authoritarian President Bashar Assad.

The government’s increasingly bloody attempts to put down the 11-month uprising have fueled mounting international criticism.

The Obama administration summoned Syria’s senior envoy in the U.S., Zuheir Jabbour, over the Homs offensive.

The State Department’s top diplomat for the Mideast, Jeffrey Feltman, expressed his “outrage over the monthlong campaign of brutality and indiscriminate shelling” in Homs, according to a statement.

U.S. Secretary of State Hillary Rodham Clinton told members of Congress on Tuesday that Assad could be considered a war criminal.

U.N. humanitarian chief Valerie Amos said Syria had not yet agreed to allow her to into the country. Susan Rice, the U.S. ambassador to the United Nations, called that refusal “shameful.”