Pakistan's political turmoil deepened on Tuesday after the two main parties in the ruling coalition split, weakening the fragile government just a week after President Pervez Musharraf resigned.

The world's only nuclear-armed Islamic nation, already facing a fresh campaign of bombings by a resurgent militant movement, now faces the prospect of a bitter political battle over the choice of Musharraf's successor.

Former prime minister Nawaz Sharif pulled his party out of the coalition on Monday, saying they were moving to the opposition because of what he said were the broken promises of the other main party's leader, Asif Ali Zardari.

He said Zardari had gone back on a pledge to reinstate dozens of judges sacked last year by Musharraf - an issue that has been at the centre of a political dispute in Pakistan for the past year.

"We have taken this decision after we failed to find any ray of hope and none of the commitments made to us were fulfilled.This situation forced us to withdraw our support," Sharif said on Monday.

Zardari, in a televised address late on Monday, appealed for Sharif's return to the government.

"We are sad over Nawaz Sharif's decision. We want to move together and solve the problems facing the nation. We will request Nawaz Sharif to return to the government," he said.

Nationwide protests

Lawyers meanwhile called for a nationwide protest on Thursday to demand the reinstatement of the judges, who were pushed out as Musharraf purged his opponents in the judiciary last year.

Sharif's PML-N party has now put forward its own candidate to challenge Zardari, widower of another former premier, Benazir Bhutto, on September 6, when lawmakers will select who will be the next president.

Political chaos is nothing new in Pakistan, which has been under military rule - including under General Musharraf - for more than half of its existence since being partitioned from India after World War II.

But the months of turmoil that eventually forced Musharraf to resign last week under threat of impeachment, and the new split between Sharif and Zardari, have made Western allies jittery about Pakistan's role in the "war on terror".

The United States, which turned Musharraf into an ally after the September 11 attacks and has supplied the country with tens of billions of dollars in aid since then, played down the importance of the split.

"I don't anticipate it would have any impact on our joint efforts to combat extremism," said US State Department spokesman Robert Wood.

The strategically important country - which has the second-largest Muslim population in the world - has seen a resurgence of Taliban and al Qaida militant activity in the lawless tribal areas along the Afghan border.

While critics have long charged that Pakistan's powerful intelligence service actually helps to support the militants, the military is nevertheless also pursuing a tough campaign against Islamist guerrillas.

Clashes in one region alone have left around 500 people dead in the last fortnight, and the Pakistani Taliban have said that the latest wave of suicide bombings will continue until the assault is stopped.

But the government said Monday it had banned the main Taliban militant umbrella group, Tehreek-e-Taliban Pakistan, and frozen its bank accounts and assets.

US diplomat attacked

Unknown gunmen opened fire on the car of a US diplomat in the northwestern city of Peshawar on Tuesday, but she escaped unhurt, police said.

The diplomat, Lynne Tracy, was on her way to her office when a Landcruiser suddenly appeared and blocked the road in the University Town neighbourhood of Peshawar, local police officer Arshad Khan said.

"A man in the intercepting Landcruiser pulled down the window and opened fire with a Kalashnikov rifle, but the diplomat remained unhurt," he said.