There has been fighting in South Waziristan between the Taliban and al-Qaeda and government forces in recent weeks, although it has subsided in recent days.
 
Several Pakistani army helicopters have crashed in the country's mountainous northwest in recent years.
 
A helicopter escorting Pervez Musharraf, the Pakistani president, to Pakistani Kashmir in October last year crashed and burst into flames, killing four soldiers.
 
The rugged region is also the hideout of Baitullah Mehsud, commander of the al-Qaeda-linked Tehreek-e-Taliban (Taliban Movement) Pakistan.
 
He is accused by Pakistani and US officials of masterminding the assassination of Benazir Bhutto, the former Pakistani prime minister.
 
Mehsud ceasefire?
 
Earlier on Wednesday, Maulvi Omar, a Tehreek-e-Taliban spokesman, said Mehsud had ordered an "indefinite" truce in South Waziristan and neighbouring regions.
 
Omar told news agencies by telephone: "Baitullah Mehsud has ordered his people to stop attacks against Pakistani forces from Waziristan to Swat and other areas of Pakistan.
 
He said "the government stopped attacking us".
 
But Abbas said the Pakistan army would keep up operations against the Taliban.
 
"There is no formal information conveyed to us from them about a  ceasefire," he said.
 
"When they stopped firing we thought it was because of the  severe weather conditions in the region."