ISLAMABAD: Under-pressure Pakistani President Asif Ali Zardari has claimed "considerable success" in a military offensive against Taliban fighters in the remote tribal northwest, his office said Sunday.

Zardari, who is battling increasing unpopularity and strained relations with the powerful military, made the remarks during a telephone conversation late Saturday with British Prime Minister Gordon Brown.

"Referring to the ongoing drive against militancy in the tribal areas of South Waziristan, the president said that considerable success had been achieved," presidential spokesman Farhatullah Babar said in a statement.

"The operation would continue till the area is cleared of terrorists and the objectives are achieved," Zardari told Brown, stressing the need for support from the international community in aiding civilians displaced by the fighting.

Pakistan sent about 30,000 troops backed by fighter jets and helicopter gunships into South Waziristan on October 17, in the most ambitious operation yet against the Taliban in their mountain stronghold near the Afghan border.

Although there has been some resistance in the region, many officials and analysts believe most of the estimated 10,000 Taliban guerrillas in the district have escaped into neighbouring Orakzai and North Waziristan.

Pakistan is also facing political uncertainty, after a legal amnesty protecting Zardari and key aides from corruption cases expired Saturday, raising fears of a fresh crisis as the country grapples with the Taliban.

Zardari on Saturday gave control of Pakistan's nuclear arsenal to Prime Minister Yousuf Raza Gilani, widely seen as a move to fend off criticism and make good on electoral promises to devolve greater power to parliament.

Zardari replaced military ruler Pervez Musharraf as president last year after his Pakistan People's Party won elections, but his approval ratings are at rock bottom as the nation struggles with Taliban violence and a recession.

Security has drastically deteriorated in Pakistan since Islamabad joined the US-led "war on terror". Hundreds of Taliban and Al-Qaeda-linked militants fled into the tribal belt after the 2001 invasion of Afghanistan.

The South Waziristan offensive, which followed a spring offensive in Swat valley, has also seen a retaliatory surge in suicide attacks targeting civilians and security officials, particularly in Pakistan's northwest.

The United States has welcomed Pakistan's military efforts but is reportedly pressuring the civilian government to also counter militants on Pakistani soil who attack NATO and US troops across the border in Afghanistan.

Babar said Brown instigated Saturday's phone call, and lauded the government's determination to fight the insurgents.

"Britain will continue to support Pakistan build its ravaged economy and in the rehabilitation of the displaced people," Brown was quoted as saying in the statement.