Tensions were running high in Yemen on Tuesday, a day after 19 anti-regime protesters were shot dead and the US administration pressed President Ali Abdullah Saleh for a peaceful transition of power.

Police shot dead two people in the Red Sea city of Hudaydah in overnight protests over the killings of 17 demonstrators earlier the same day in Taez, south of the capital.

Security forces in Taez opened fire after protesters demanding Saleh's ouster marched on the governorate headquarters in the city, about 200 kilometres (125 miles) from Sanaa.

Witnesses said security forces were deployed in strength in the streets of both Taez and Hudaydah on Tuesday, as anti-regime protests which first broke out in late January, showed no sign of abating.

But Washington, which considers Saleh a key ally in its "war on terror" but has expressed fears of Al-Qaeda taking advantage of a power vacuum, is now pressing the long-time leader to negotiate a peaceful transition of power.

"We are obviously concerned that in this period of political unrest that Al-Qaeda and other groups will attempt to take advantage of that power vacuum," White House spokesman Jay Carney said on Monday.

"That is one of the reasons why we urge political dialogue to take place and a timetable for this transition that President Saleh has talked about to be begun," Carney said.

Saleh's official response to opposition demands to step down and hand over to his deputy for an interim period has been to urge protesters to go home and dismantle their roadblocks.

He said, in a vague statement, that he was willing to discuss a "peaceful transition of power through constitutional ways".

The autocratic leader, in power since 1978, had said he was willing to step down by the end of this year, but his ruling General People's Congress party has defiantly said he should serve out his term until 2013.

The White House fielded questions after the New York Times reported that the administration had shifted its position and concluded Saleh will not bring about reforms and now needs to be eased out of power.

Yemen under Saleh has been a vital US partner in cracking down on Al-Qaeda in the Arabian Peninsula, which has been blamed for several thwarted attacks directed at the US mainland.

"We believe we can and will work with the government of Yemen on these important matters," Carney said, but significantly did not lock US cooperation onto Saleh.

"We are not focused on one individual," he said.

A State Department spokesman repeated that the outcome of the crisis was in the hands of "the Yemeni people", adding Washington was continuing to consult with both sides.

"There's a gap between what President Saleh said and what the people have asked for," spokesman Mark Toner said. "In our discussions with the government and with the opposition ... we're helping or talking about bridging that gap."

Meanwhile, foreign ministers of oil-rich Gulf states have said they are seeking to mediate in Yemen.

An unnamed Yemeni official on Tuesday welcomed a reported invitation to representatives of the government and the opposition to hold talks in Riyadh, according to state news agency Saba.

More than 100 people have been killed in Yemen's crackdown on protesters.