Sanaa, Yemen (CNN) -- Tribesmen intent on ending the regime of President Ali Abdullah Saleh fought Friday with government forces, resulting in at least four deaths in the city of Taiz, according to medical staff in Freedom Square.

Another 17 people were wounded in the violence, the medical staff said.

North of Taiz in the capital city of Sanaa, peaceful pro- and anti-government protests were held after Friday prayers, with many thousands of people in attendance at each event.

They occurred a day after a U.S. drone strike targeting militants in southern Yemen killed at least 50 people, two Yemeni security sources said.

The United States and the Yemeni government have stepped up their efforts to target militants, including those Islamists who've taken over several cities in recent weeks.

The government said that a U.S. drone was not involved in the attack and that its air forces conducted the raid. The Interior Ministry said on its website that nine fighters were killed and dozens were wounded and that the number of deaths was expected to rise.

There was no immediate comment from U.S. officials.

Both sources, a security official and a senior security source, didn't want their names used because they are not authorized to speak to the media.

The airstrike occurred in al-Wathee district in Abyan province. One of the sources said more than a dozen people were wounded.

The strike targeted a police station that had been taken over by suspected al Qaeda fighters, the sources said. U.S. drones had been seen flying over the area in recent days, and more attacks were expected, the sources said.

At least seven vehicles and other equipment belonging to the fighters were destroyed.

"The casualty toll is high because fighters were gathered in that area with family members," said the senior security source in Abyan.

Two witnesses said that at least 30 civilians who had been hiding from the attacks were among the dead.

"No one knows who is dying in Abyan," said Yousra Bandar, a mother of three. "We want to leave the province, but go to where? Leaving the province is a slow death for all of us."

Two years ago, a U.S. drone attack in Abyan killed 62 people.

There has been instability in the province, with an Islamist extremist group called Ansar Sharia fighting the government since May.

Saleh remains in Riyadh, Saudi Arabia, where he is recovering from a June 3 attack on his compound that left him with extensive burns on his face and body.

He will return to his country Sunday, a homecoming that also marks the 33rd anniversary of his rule, a senior ruling party official said Wednesday.

The official, a political adviser for Saleh's ruling General People Congress Party, said that the president's health was improving quickly and that Yemenis would celebrate his return.

He said that a massive celebration is being planned for the president's homecoming and that Saleh will decide upon his return "what is best for Yemen and the ruling party."

But even during his treatments -- which included eight surgeries -- Saleh has been under pressure to embrace a political transition plan developed by the Gulf Cooperation Council that included plans for him to step down following months of widening anti-government protests and sentiment in his country.

Officials in Sanaa have rejected calls for Saleh to leave office, saying that no power has the authority to force Saleh to step down.

John Brennan, U.S. President Barack Obama's top counterterrorism adviser, has urged Saleh to sign the Gulf Cooperation Council's political transition plan.

Saleh has voiced agreement with the plan, which would ensure his gradual departure from office, but he has not signed it. Saleh told Brennan that the initiative laid the groundwork for exiting the political crisis through national dialogue involving all political parties, Yemen's state-run Saba news agency reported.

Ahmed Bahri, head of the political department at the opposition Haq party, has accused the United States of doing too little. "The Yemeni revolution vowed to stay peaceful, but now is the time to review that stance," he said. "The use of force might be needed to oust the Saleh regime from power."