Pakistan's top court on Thursday heard an appeal from the prime minister against a summons to face being indicted for contempt, in a saga that could force his weak government into early elections.

The Supreme Court summoned the embattled Yousuf Raza Gilani to appear next Monday to face indictment over the government's two-year refusal to ask Swiss authorities to re-open graft cases against President Asif Ali Zardari.

It was a shock move that sharply escalated pressure on Gilani, who if convicted could be jailed for six months and disqualified from public office.

Many observers believe that pressure from the courts could ultimately force early elections on an unpopular government, a year away from becoming the first in the history of Pakistan to complete an elected term in office.

Chief Justice Iftikhar Muhammad Chaudhry led the hearing, but the eight-judge bench appeared intransigent that the government must respond to its request in December 2009 to re-open corruption cases.

Zardari and his late wife, prime minister Benazir Bhutto, were suspected of using Swiss bank accounts to launder about $12 million in alleged bribes paid by companies seeking customs inspection contracts in Pakistan in the 1990s.

Chaudhry insisted the prime minister should have taken the initiative in re-opening those cases. The court said it had shown patience in the two years and three months since it ordered that the matter be taken up with the Swiss.

Gilani's lawyer Aitzaz Ahsan pointed out that the Swiss authorities had themselves closed the cases in 2008, when Zardari took office, on the grounds of immunity, and that there was no third party to claim the funds.

"The prime minister should not be the person to undermine any institution. This money will not come to our pockets. Actually it is the nation which wants this money," Chaudhry told the court.

Gilani himself appeared before the Supreme Court on January 19, refusing to back down and citing Zardari's immunity as explanation for his refusal to obey the court's order.

Legal experts say that Gilani can only avoid being charged by lodging a successful appeal, apologising or promising to write to the Swiss.

The allegations against Zardari, so tainted by corruption allegations that his nickname is "Mr 10 Percent" were frozen by a Pakistani political amnesty imposed in 2007, which the courts overturned in late 2009.

The president has already spent 11 years in jail on charges ranging from corruption to murder, although he was never convicted and his supporters say that the charges were politically motivated.

A Swiss prosecutor says it is impossible to reopen the case against Zardari because he benefits from immunity as a head of state.

Ahsan said Wednesday that his appeal was based on precedents set by courts in Australia, Britain, France, India and the United States.

Members of the government accuse judges of plotting with the army to wage a witch-hunt against the prime minister and president.