ISLAMABAD: Pakistan headed into fresh turmoil Sunday with President Asif Ali Zardari locked into a humiliating confrontation with the country's top judicial authority over the appointment of senior judges.

The showdown threatens Zardari's weak government at a time of mounting US pressure on the nuclear-armed country to eliminate Taliban and Al-Qaeda-linked militants as Washington seeks to end the war in neighbouring Afghanistan.

The crisis arose when the president appeared to ignore a candidate nominated by Chief Justice Iftikhar Mohammad Chaudhry to the Supreme Court, instead selecting another senior judge.

The Supreme Court suspended the president's decree appointing Khawaja Sharif, the top judge in Pakistan's second biggest city Lahore, and a judge to fill Sharif's shoes, and summoned the attorney general to appear in court.

"We are heading towards a very serious situation. If it was proved that the president violated the constitution then under article 177 the Supreme Court can disqualify the president," senior lawyer Qazi Anwar told AFP.

Pakistani political analyst Hasan Askari said the latest showdown was the climax of a crisis that had been brewing for almost a year.

"If the president doesn't accept this ruling from the Supreme Court then Pakistan has a very serious constitutional crisis," he told AFP.

Zardari, so tainted by corruption that his nickname is "Mr Ten Percent" despite his 2008 election on a wave of sympathy over the assassination of his wife ex-prime minister Benazir Bhutto, has found a nemesis in Chaudhry.

The chief justice, independent-minded and extremely popular in Pakistan, was sacked in October 2007 by then military ruler Pervez Musharraf.

Zardari reinstated him last March, but the move was seen as an embarrassing climbdown on the eve of a protest march on the capital Islamabad whipped by the political opposition and under Western pressure to avert a political crisis.

On December 16, the Supreme Court abolished a decree protecting Zardari and other government figures from prosecution, exposing the president to the possibility of having his immunity and eligibility for office challenged.

Analysts since suggested the Supreme Court was stepping back from pursuing Zardari and the popular opposition party, the Pakistan Muslim League-N (PML-N), failed to jump on the bandwagon.

But Zardari's controversial appointment of Sharif late Saturday put him back on open collision with Chaudhry, who had instead proposed Justice Saqib Nisar for the seat.

"The president is pleased to appoint Mr Justice Khawaja Muhammad Sharif, chief justice of Lahore High Court as judge of Supreme Court of Pakistan with immediate effect," Zardari's office had announced.

It took barely four hours for the Supreme Court to clip the president's wings.

"The appointment of Mr Justice Khawaja Muhammad Sharif as a judge of the Supreme Court, prima facie, appears to have been issued in violation of the provisions of the Constitution... hence the same is suspended," it said.

The Supreme Court summoned the attorney general to appear at a hearing adjourned until February 18, senior lawyer Hamid Khan told AFP.

Information Minister Qamar Zaman Kaira released a curt statement through state media saying that Zardari made the appointments "as per constitutional provisions" and "after thorough consultations."

Analysts said the Supreme Court and Zardari could each argue they were in the right, but warned the crisis could bring down the entire government if allowed to escalate.

Media reports that Zardari could hit back by declaring a state of emergency, as Musharraf did in 2007, were reportedly denied by his office and most analysts believe the only recourse could be to execute an embarrassing U-turn.

"The final interpretation of the constitution is done by the Supreme Court," said Askari, warning that Saturday's suspension order had already pushed the president into "an extremely difficult and humiliating situation".

"If the Supreme Court goes ahead and tries to pull him down, then perhaps the system will collapse... and perhaps there will not be constitutional government in Pakistan," Askari warned.

Lawyers said Zardari had to back down, but rubbished any possibility of military intervention given that Pakistan only emerged from military rule two years ago.

"He is already besieged and if he does not step back it will be a very serious situation for him," said Anwar.