(CNN) -- Yemeni protesters and military and pro-government gangs clashed in several areas Tuesday, with at least six killed and hundreds more injured, as the future of President Ali Abdullah Saleh remained uncertain.
The United States has no intention of stopping its military aid to Yemen, despite the unrest, Pentagon spokesman Geoff Morrell said Tuesday. The aid, in support of Yemeni counterterrorism efforts, continues to be essential because of the "real threat" from al Qaeda in the country, he said.
In Sanaa, the capital, eyewitnesses and field medical teams told CNN that security forces and anti-riot police used batons to attack protesters among 40,000 people marching on Zubairy Street Tuesday evening. In addition, pro-government gangs attacked protesters on Tuesday near a military base.
Four people were killed -- three pro-government demonstrators and one anti-government demonstrator. Windows were shattered on an ambulance carrying some of the 56 injured protesters to a hospital, witnesses said.
"The government forces are killing us," said Abdullah Salem, a youth activist who was at the protest. "Saleh and his militia will not succeed, and every blood spilt will be accounted for in international courts."
In the city of Taiz, meanwhile, at least two anti-government protesters were killed when security forces and Republican Guards fired on protesters, according to medical teams. Hundreds of people were injured, 55 of them from gunshot wounds.
The security chief in Taiz denied his forces fired on demonstrators.
"Security forces did not attack protesters," said Abdullah Qiaran. "We were dispersing pro and anti-government protesters after we saw that both sides were clashing."
An estimated 30,000 demonstrators marched near the presidential palace in the port city of Hodeida Tuesday evening, witnesses said.
The violence comes as the United States is helping to mediate a transition out of office for Saleh, who has been facing popular protests for weeks, according to two Yemeni officials.
The timing is delicate, officials said, because they want to avoid any steps that could further destabilize Yemen or interrupt counterterrorism efforts in a country that U.S. officials believe is home to some of the world's most active al Qaeda operatives.
The White House said in a statement Tuesday it "strongly condemns the use of violence by Yemeni government forces against demonstrators in Sanaa, Taiz and Hodeida in the past several days. The Yemeni people have a right to demonstrate peacefully, and we remind President Ali Abdullah Saleh of his responsibility to ensure the safety and security of Yemenis who are exercising their universal right to engage in political expression."
The statement called upon Yemen's government to investigate and hold those responsible accountable.
"The United States strongly supports the Yemeni people in their quest for greater opportunity and their pursuit of political and economic reform that will fulfill their aspirations," the statement said. "President Saleh needs to resolve the political impasse with the opposition so that meaningful political change can take place in the near term in an orderly and peaceful manner."
On Sunday, the state-run Saba news agency reported clashes there caused the deaths of 10 people and injured a number of other demonstrators. According to Saba, witnesses said the opposition had deployed armed militias in the city.
Taiz Gov. Hamoud al-Soufi said protesters had attacked the governor's building and the presidential palace in Taiz, and that he was forming a committee to investigate the incident.
"We express our deep regret for the deviation of peaceful protests," he said.
The U.N. human rights office on Tuesday called on Yemen to halt the use of force against protesters, citing more than 100 deaths since the protests began.
British Foreign Secretary William Hague also said he was appalled by the reports of further deaths this week.
"In early March, President Saleh promised to maintain maximum restraint in the use of the Yemeni security forces in controlling peaceful demonstrations. We strongly urge him to keep that promise," he said.
Earlier, an Obama administration official told CNN that the United States has been working behind the scenes, trying to determine the best way forward.
"Whatever timeline they develop, we can accept -- but it needs to be worked out. That's the problem that's making people anxious," he said. "There has to be a path forward, it can't be a situation where he thinks that, week to week, he can sort of continue to hold on without making some kind of accommodation."
Saleh offered to step down by the end of the year after constitutional reforms and new elections, but the opposition, led by the Joint Meeting Parties bloc, is demanding he leave immediately. A plan unveiled by the bloc Saturday called for Saleh to hand over all authority to Vice President Abdu Rabu Mansour Hadi in the interim. But Saleh is not offering any further concessions, and protests by the opposition continue.
U.S. officials said various groups, including al Qaeda and secessionist elements, are exploiting the political turbulence and splits within the military and security services for their own gain.
The rapidly deteriorating situation presents a major problem for the Obama administration.
Saleh has been in power since 1978 and has been a staunch U.S. ally in the fight against al Qaeda in the Arabian Peninsula. Saleh argues he should remain in office because he is the only one who can effectively continue that fight against terrorism.
One U.S. official said Yemen presents a serious challenge for the administration, as the government is "preoccupied with political unrest," and little is being done on the counterterrorism front.
A counterterrorism official said there is concern over Yemen's pursuit of terrorists. "The ability of what's left of the government to manage internal security which includes (al Qaeda) is in question," said the official. A weakened security system in the country "provides an opportunity to be exploited" by al Qaeda, the official said.
CNN's Mohammed Jamjoom, Jill Dougherty, Pam Benson and Elise Labott contributed to this report