* Heavy clash in South Waziristan
* Swat operation seen over in "two or three days"
* Army urges civilians to leave Swat town
By Kamran Haider
ISLAMABAD, May 31 (Reuters) - Pakistani forces battled militants in South Waziristan on the Afghan border on Sunday as a government official said an offensive in the Swat valley could be over in two or three days.
Pakistani forces have undertaken their most concerted offensive against an expanding Taliban insurgency that has raised fears for the nuclear-armed U.S. ally’s stability and the safety of its nuclear arsenal.
The focus of the fighting has been the former tourist destination of Swat, 120 km (80 miles) northwest of Islamabad, which the Taliban virtually took as the government alternated between inconclusive military action and peace pacts.
But tension has also been rising in South Waziristan, an al Qaeda and Taliban stronghold, with military officials saying an offensive was likely there after Swat is secured.
The United States and the Afghan government have long been pressing Pakistan to root militants out of South Waziristan and other enclaves on the Afghan border, from where the Taliban direct their Afghan war.
Militants attacked a paramilitary force camp in Jandola, 80 km (50 miles) east of Wana, the main town in South Waziristan, late on Saturday, security officials said.
"Militants came in force and attacked a paramilitary camp and fighting lasted for eight hours. At least 40 militants were killed while four soldiers died," said an intelligence official in the region who declined to be identified.
A military spokesman said the militants had been pushed back after a heavy exchange of fire. Up to 15 militants and three soldiers were killed, he said.
There was no independent confirmation of the casualty estimates.
Militant violence in Pakistan has surged since mid-2007, with attacks on the security forces, as well as on government and Western targets.
There have been eight bomb attacks in various towns and cities since the offensive in Swat and neighbouring districts began in late April and the Taliban have threatened more.
The offensive in Swat has sparked an exodus of about 2.4 million people, according to government figures, and the country faces a long-term humanitarian crisis.
Bomb attacks in cities and the plight of the displaced could undermine public support for the offensive but for now, analysts say, the authorities are determined to defeat the Taliban in Swat.
The army said on Saturday it had regained full control of Mingora, the main town in Swat, and a top Defence Ministry official said on Sunday the military operation could be over in a two or three days.
"Only five to 10 percent of the job is remaining and hopefully within two to three days, the pockets of resistance will be cleared," Syed Athar Ali, secretary of defence for Pakistan, said at a regional defence meeting in Singapore. [ID:nSP214569]
Military spokesman have been cautious about predicting how long the offensive would last, saying there was still resistance in the valley.
"It’s very difficult to give a timeline," said chief military spokesman Major-General Athar Abbas. "It’s a very big area so nobody’s in a position to give any timeline for the operation."
On Sunday, the military urged civilians to leave the town of Charbagh, about 15 km (10 miles) north of Mingora, and lifted a curfew there and in Mingora to allow people to get out.
Pakistan is vital for U.S. plans to defeat al Qaeda and cut support for the Afghan Taliban.
The United States, which is sending thousands of reinforcements into Afghanistan, has been heartened by the offensive in Swat. (Writing by Robert Birsel; Editing by Bill Tarrant)