Suspected Al-Qaeda gunmen killed four soldiers in Yemen on Sunday, a day after President Ali Abdullah Saleh refused to yield to protesters demanding his immediate resignation.
The new violence came as both London and Washington called on their citizens to consider leaving the impoverished Arabian Peninsula country and warned against all but essential travel.
The elite Republican Guard soldiers were ambushed as they delivered food near Marib about 170 kilometres (110 miles) east of Sanaa, a local official said. "The attack was similar to others by Al-Qaeda," the official added.
A military attack helicopter and ground troops pursued the assailants into a nearby valley, a security source said.
Yemen is a key US ally in the fight against Al-Qaeda's branch in the Arabian Peninsula, which a state department official last month described as the "most significant" threat to the US homeland.
Sunday's attack was not believed to be connected to the anti-government unrest, which has killed at least 19 people in Yemen since 16 February, according to an AFP toll.
Security forces arrested 16 protesters in the main southern city Aden on Saturday as thousands continued to demonstrate, demanding Saleh's resignation over corruption, poverty and high unemployment in one of the poorest countries in the world.
But the veteran leader, who has ruled since 1978 dismissed an opposition proposal for a quick transition of power, saying he would serve out his current term, which expires in 2013.
In a statement carried late Saturday on the state-run Saba news agency, an official close to the president said the opposition's proposal, envisaging Saleh's departure before the end of the year, was "vague and contradictory."
"A peaceful transition of power cannot be done amidst chaos, but by having recourse to the people through elections, so that they can decide who they want to lead without acts of violence and trouble," the statement said.
Yemen's opposition and clerics last week offered Saleh a smooth exit from power this year, even as protests calling for his immediate removal spread from the south to the east of the country.
The five-point proposal calls for a "peaceful transition of power" from Saleh and insists demonstrations against his regime will continue. It demands a probe into a deadly crackdown on the protests.
Once Saleh replied, it would ultimately be up to Yemen's people to "decide whether to accept or reject this proposal," said the Common Forum, an alliance of parliamentary opposition groups.
Witnesses said police used tear gas and fired warning shots on Saturday to disperse the protesters and that two demonstrators were wounded after being beaten with batons.
Meanwhile, thousands of anti-government protesters took to the streets in the city of Ataq in the eastern province of Shabwa on the third consecutive day of protests, witnesses said.
"The people want to topple the regime," demonstrators chanted, echoing a slogan that has gripped many Arab capitals and that has already forced the presidents of Tunisia and Egypt to quit and has thrown Libya into civil war.
A minister of parliament from neighbouring Al-Bayda province announced Friday his resignation from the ruling party of Saleh in protest against the use of force against demonstrators.
Ali al-Umrani announced his decision to quit the General People's Congress and join anti-government protests at an anti-Saleh demonstration in the capital, Sanaa.
Another member of the GPC, prominent businessman Nabil al-Khameri, also announced his resignation to protest the violence.
Eleven MPs who quit the GPC last week have since announced the formation of a new parliamentary bloc called the "Free Deputies," headed by Abdo Bisher.
The US government warned Americans Sunday against travelling to Yemen and authorised the voluntary departure of family members and non-essential embassy staff.
Britain on Saturday advised its nationals against all travel to Yemen.