A walkout of medical staff throughout Gaza has strained services at hospitals and clinics throughout the territory, the latest in a series of crippling strikes that are deepening bitter divisions between Gaza's militant Hamas rulers and loyalists of moderate Palestinian President Mahmoud Abbas.

The strike has forced non-complying doctors to pull double shifts and left residents struggling for treatment, adding to the hardships in a territory suffering from international isolation since Hamas wrested control of Gaza from Fatah-allied security forces in June 2007.

The Medical Workers' Union, dominated by Abbas' Fatah movement, called the strike last week to demand Hamas reinstate workers Fatah says have been fired for their political loyalties. The union said Hamas police have forced some essential staff to report to duty under the threat of arrest.

Hamas has accused Fatah of calling the walkout at state-run hospitals and clinics as a political ploy _ but has aggravated the crisis by shutting down private clinics run by striking doctors.

Around half of Gaza's thousands of doctors, nurses and administrative medical workers have walked off the job since last week, according to the World Health Organization. Hospital workers carried out a similar strike last year after Hamas fired several senior doctors, but the doctors weren't rehired and the slowdown fizzled after five days.

Gaza hospitals are barely grinding along because working doctors and nurses are putting in 12-hour to 24-hour shifts and running on skeleton staff, many only treating emergency cases. Many clinics are closed.

"I want to ask something. The doctors and nurses that went on strike, if their son or brother is sick, will they watch him die?" asked Mohammed Musa, an unemployed, 36-year-old father who could not find a doctor to treat his son's fever.

Union leader Osama Najjar said hundreds of medical staff were called in for interrogations, and dozens of pro-Fatah employees have been fired and replaced with Hamas loyalists.

Mahmoud Zahar, a top Hamas leader and physician by training, said the government had to take action because key staff were not turning up. He said there isn't enough medical staff in Gaza to replace strikers, prompting police to force them to work.

"We can replace a teacher but we can't replace a doctor. Where will we get them from?" he asked.

Last week, the pro-Fatah teachers' union went on strike, also claiming that Hamas was filling key positions with its cronies. The strike disrupted the start of the school year. Hamas reacted by firing some 2,000 teachers and replacing them with Hamas supporters.

"Anyone who continues observing the strike is going to be penalized and might be fired," said Khaled Radi, spokesman for Gaza's Hamas-run Health Ministry.

The independent Palestinian Center for Human Rights said the Abbas-led Palestinian Authority, which rules the West Bank, was behind the strike. Although the authority no longer controls Gaza, it still pays the salaries of thousands of civil servants there and has threatened to cut the wages of workers who don't join the strike.

Nimr Hamad, an Abbas adviser, said the claims were not true. But there has been a precedent of Ramallah meddling in Gaza's medical sector: Last year, the West Bank government ordered a work slowdown to protest the arrest of a prominent physician allied with Fatah.

Abbas wants to regain control of Gaza and incorporate it into a future Palestinian state. The West Bank and Gaza are located on opposite sides of Israel.

One nurse, a Fatah loyalist who administers vaccinations, said he felt pressure to join the strike, even though he thought it was unethical, because he feared his salary would be cut.

"If a child doesn't take a vaccination, it could have dangerous side effects," he explained. He asked not to use his name, fearing retribution by the West Bank government.

The United Nations warned the strike was harming Gaza's most vulnerable residents and deepened divisions between Gaza and the West Bank.

"These actions and the subsequent strike called by unions threaten the provision of health and education services to the people of Gaza who already face considerable hardship," said U.N. Middle East peace envoy Robert Serry.