* 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 and a government official said an offensive in the Swat valley could be over in two or three days.
Pakistan has been carrying out its most concerted offensive against an expanding Taliban insurgency, which has raised fears for the stability of the nuclear-armed U.S. ally 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 was 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 near the town of Jandola, 80 km (50 miles) east of Wana, the main town in South Waziristan, late on Saturday, security officials said.
"They carried out a very serious attack on our positions at around midnight. It was repulsed after a heavy exchange of fire," said military spokesman Major-General Athar Abbas.
Up to 15 militants and three soldiers were killed, he said, although an intelligence official in the region said earlier at least 40 militants and four soldiers were killed. There was no independent confirmation of the casualty estimates.
Militant violence has surged in Pakistan 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. The United Nations has pleaded for contributions for a $543 million fund to help.
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 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 spokesmen have been cautious about predicting how long the offensive would last, saying there was still resistance.
"It’s very difficult to give a timeline," said Abbas. "It’s a very big area so nobody’s in a position to give any timeline for the operation."
The military says 1,217 militants have been killed since late April, while 81 soldiers have been killed and 250 wounded. There are no independent casualty estimates available.
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 and thousands of people left the two towns.
"We have to flee. I don’t know what will happen tomorrow," Mingora resident Mohammad Nisar told Reuters.
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. (Additional reporting by Hafiz Wazir, Alamgir Bitani and Junaid Khan; Writing by Robert Birsel, Editing by Dean Yates)