SANAA (AFP) - Yemen's opposition movement vowed on Sunday to intensify protests against the regime of President Ali Abdullah Saleh, after the embattled leader refused to resign by the end of the year. With violence gripping the strategic US ally on multiple fronts, Washington and London advised their citizens to consider leaving the Arabian Peninsula nation. The opposition Common Forum called on protesters to step up demonstrations which have left at least 19 people dead since January 27, according to an AFP toll. Amnesty International counts at least 27 dead. "We have called upon the people to widen demonstrations and escalate the peaceful struggle in all regions until he (Saleh) is left with one option, that is to leave," said Mohammed Sabri, a leading member of the forum. The ultimatum came a day after Saleh - who has ruled the deeply tribal nation since 1978 - dismissed opposition calls for his resignation by the end of the year and vowed to serve out his current mandate until 2013. In a statement carried late Saturday on the state-run Saba news agency, an official close to Saleh said an opposition transition plan envisaging his departure before the end of this year was "vague and contradictory." "A peaceful transition of power cannot be done with 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. Sixty-one anti-regime protesters were wounded on Sunday after supporters of the ruling General People's Congress party armed with knives, rocks and batons stormed a protest in the city of Ibb, south of Sanaa, leading activist Abdulkarim Mohammed Ali said. Four elite Republican Guard soldiers were shot dead as they delivered food near Marib, about 170 kilometres (110 miles) east of Sanaa, an official told AFP. In the south, Al-Qaeda gunmen shot and killed an army colonel who was shopping at a market in Zinjibar, Abyan province, a security official said. Another officer was later shot dead at a restaurant in the city of Sayun, in the southeastern province of Hadramut. Yemen is a key US ally in the fight against Al-Qaeda in the Arabian Peninsula (AQAP), which a State Department official last month described as the "most significant" threat to the US homeland. Sunday's attacks were not known to be connected to the anti-government unrest, but were a reminder of what is at stake if Saleh is ousted and a power vacuum ensues. A State Department travel advisory issued Sunday said US citizens in Yemen "should consider departing" and described the security threat as "extremely high due to terrorist activities and civil unrest." "The US embassy's ability to assist US citizens in the event of a crisis in Yemen is very limited," it warned. The British Foreign Office issued a similar advisory, warning of the "threat of terrorism, kidnapping and tribal violence."