KARACHI, Pakistan (AP) _ A Greek-registered oil tanker that ran aground off Pakistan's coast broke apart Thursday, and clean up crews were working to prevent a major oil spill, officials said.

It wasn't immediately clear how much crude leaked out of the MT Tasman Spirit when the vessel's cracked hull split open near the southern port of Karachi, said Iftikhar Rashid, federal communications ministry secretary.

About half of the ship's oil had been siphoned away since it became stranded in monsoon rains and rough seas two weeks ago in the Arabian Sea, Rashid said. But 35,000 metric tons (38,600 US tons) of oil were left in its tanks, he said.

``The ship has broken apart in two, but we have started stabilizing it,'' Rashid told reporters.

The tanker, chartered by Pakistan National Shipping Corp., was carrying oil for the state-run Pakistan Refinery Ltd.

Some doctors feared the crude oil could become harmful and toxic if it gets oxidized in the water or in the air of Karachi _ Pakistan's most populous city.

``This may cause not only nausea, headaches and throat infections, but may also hurt the human brain, especially when one is exposed to toxins for a long time,'' said Dr. Wafay Shakir, a neurosurgeon at a Karachi hospital.

Residents with homes near the sea were already reporting having nausea and headaches.

``Both my son and wife started vomiting and complaining of headaches because of the smell from the beach,'' said Anwar Ahsan, a local banker who lives in the Sea View apartment blocks that overlooks the Arabian Sea.

Driven away by the smell, Ahsan moved his family to stay with relatives elsewhere in the city.

But government officials said residents were not in danger.

``We have talked to environmental experts and they say that there is no threat to people until they enter the sea water,'' Rashid said.

British and Singaporean ships arrived at the Karachi port on Wednesday to help with the cleanup. They carried special equipment to decontaminate the waters and prevent the spill from spreading further, said Brig. Javed Iqbal Cheema, head of Pakistan's National Crisis management cell.

Pakistan's neighbor and biggest rival, India, also offered help.

``As a gesture of cooperation, the government of India today offered assistance to Pakistan to deal with environmental and other hazards created by the oil spill of the Karachi port,'' said Indian External Affairs Ministry spokesman Navtej Sarna.

Pakistan did not immediately comment on the offer.

All public beaches in Karachi have been shut down, and some residents complained the move spoiled their celebrations Thursday of Pakistan's Independence Day.

Sohail Wazir Ali, who was trying to get to the beach, was stopped by police and his motorcycle confiscated.

``We celebrate Independence Day every year by going to the beach and now the police are stopping us,'' Ali said, adding that he was not concerned by the oil fumes. ``It doesn't matter because we breathe in the city pollution and fumes every day.''