Police and armed men in civilian clothes opened fire on anti-government demonstrators in the Yemeni cities of Taiz and Hudaida on Monday, witnesses said, as a drive to oust President Ali Abdullah Saleh gathered pace.

The attempt to suppress mounting protests inspired by uprisings in Egypt and Tunisia came amid signs that the United States is seeking an end to Saleh's 32-year rule, long seen as a rampart against Yemen-based Al Qaeda in the Arabian Peninsula.

In Taiz, south of the capital Sanaa, police shot at protesters trying to storm the provincial government building, killing at least 15 and wounding 30, hospital sources said.

"The regime has surprised us with this extent of killing. I don't think the people will do anything other than come out with bare chests to drain the government of all its ammunition," parliamentarian Mohammed Muqbil al-Hamiri told Al Jazeera.

Television showed a row of men, apparent tear gas victims, lying motionless and being tended by medics on the carpeted floor of a makeshift hospital in Taiz.

In the Red Sea port of Hudaida, police and armed men in civilian clothes fired live rounds and tear gas at hundreds of demonstrators marching on a presidential palace, medical sources said. Three people were hit by bullets, around 30 were stabbed with knifes, and 270 were hurt from inhaling tear gas.

As opposition forces stepped up their actions, Saleh again appeared defiant.

"Just as you gave us your confidence, we will respond to that. We will be steadfast like the mountains," he told hundreds of tribesman who chanted their rejection of concessions. "We will stay loyal to you, just as you have been loyal to constitutional legitimacy."

Saleh has said he will not run for re-election in 2013 and could step down following new presidential and parliamentary elections within a year. On Sunday, he called on the opposition to end protests to help ease talks.

An opposition proposal would see the army and security forces restructured by a vice-president acting as temporary president ahead of political reforms and elections. The ruling party says Saleh should remain in office to oversee changes.

There were signs of mounting U.S. pressure on Saleh to go. The New York Times said on Monday Washington had "quietly shifted positions" and "concluded that he is unlikely to bring about the required reforms and must be eased out of office."