Government supporters wielding knives and handguns attacked protesters in southern Yemen on Sunday, leaving one dead in the latest in weeks of demonstrations demanding the president step down.
In a separate development in the increasingly chaotic Middle Eastern nation, suspected al-Qaida gunmen killed four soldiers from the elite Republican Guard forces in a mountainous region.
Even before Yemen was hit by the wave of protests that began in mid-February, the country's government was weak and struggling to contain one of the world's most active al-Qaida branches, a secessionist rebellion in the south and a Shiite uprising in the north.
Then protesters caught a wave of unrest that is sweeping many Arab nations, calling for President Ali Abdullah Saleh to step down after 32 years in power, a demand he has repeatedly rejected while trying to assuage opposition groups.
Saleh has said he would not seek another term in office in 2013, and offered to form a unity government with opposition figures, but the overtures have failed to appease the protesters.
Eyewitnesses said by telephone Sunday that "government thugs" marched toward the protesters in the southern province of Ibb without being stopped by security forces. They spoke on the condition of anonymity for fear of government reprisal.
One protester was killed and 37 others were injured in an attack that took place on the main square, where protesters have been camping for a week.
The clashes came after some 200,000 protesters in Ibb, 190 kilometers (120 miles) south of Sanaa, took part in a demonstration Friday calling on Saleh to step down.
Meanwhile, security officials said the suspected al-Qaida gunmen ambushed the soldiers while traveling in a vehicle to distribute food to fellow troops manning checkpoints. They said the assailants managed to flee.
Al-Qaida militants have frequently attacked army soldiers in the central Marib province, an al-Qaida stronghold and one of several areas where the central government has little authority.
The officials spoke on condition of anonymity because they are not authorized to brief the media.
Al-Qaida's activity has grown in Yemen over the last couple years, with several foiled terror attacks in the United States and elsewhere traced back to operatives based here.
Citing terrorism and civil unrest, the U.S. government on Sunday advised Americans not to travel to Yemen and said Americans in the impoverished Arab nation should leave.
The State Department also authorized family members of U.S. Embassy staff and nonessential personnel to leave, saying the threat level in the country was "extremely high." The British government issued a similar warning to its citizens on Saturday.