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 doctors 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 TV.

The 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 attacked a march towards a presidential palace.

Three people were hit by bullets, around 30 were stabbed with knifes, and 270 were hurt from inhaling tear gas, doctors said.

Later on Monday, doctors said at least six demonstrators were shot dead and several wounded during evening rallies, and that the toll was likely to rise.

In Washington, the US State Department called the latest violence in Yemen "appalling".

Yemen's opposition coalition appealed in a statement to the United Nations, human rights groups and other international bodies "to intervene quickly to stop President Saleh and his entourage from shedding more blood".

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.

Gulf Arab states have invited Yemeni government and opposition representatives to talks in Saudi Arabia, Kuwaiti Foreign Minister Sheikh Mohammad al-Salem al-Sabah said, in a bid to end the crisis, but a date is yet to be set.

"On Yemen, there are some ideas that will be addressed to the Yemeni sides. I don't want to use the word mediation because now we are in a stage of feeling the pulse," UAE Foreign Minister Sheikh Abdullah bin Zayed al-Nahayan said on Monday.

There were signs of mounting US 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".

US role

Analysts and diplomats predicted more behind-the-scenes pressure on Saleh from Western countries to end the crisis.

"The next step is putting aid to Yemen on the table and saying that there are going to be serious consequences if Saleh continues to use violence against his own people," said Shadi Hamid, an analyst at the Brookings Doha Centre in Qatar.

The Obama administration has not so publicly urged Saleh to step aside. Such calls were key in bringing an end to the rule of Tunisia's Zine al-Abidine bin Ali and Egypt's Hosni Mubarak.

Sources close to the talks have said Washington gave Saleh an ultimatum last week to agree on a deal negotiated by the US ambassador in Sanaa to ensure a peaceful exit and transition of power, otherwise it would publicly call on him to step down.

Opposition sources say talks have stalled because Saleh is manoeuvring to ensure he and his family do not face prosecution over corruption accusations raised by the opposition.

A diplomat in Sanaa said the focus for now was still on talks and public calls to stand down -- which have only so far come from France -- were premature.

"It depends on developments in the coming days. This is one of the options that all capitals have if they want," he said.

"At the moment, diplomatic parties are working behind the scenes to encourage an agreement on political transition between Yemeni parties. Other options are being kept at the moment in the drawer," the diplomat said.

If Washington were to call on Saleh to go, "I'm not sure if he (Saleh) would immediately cave in," he added. – Reuters