The army launched the long-awaited biggest offensive in the tribal region against local and foreign militants June 15 after the fragile peace dialogue deadlocked after deadly Taliban attacks.
Military officials say the security forces have killed more than 500 militants and destroyed their command and control system in the volatile tribal region.
As the Taliban have in the past carried out revenge attacks, the government of Prime Minister Nawaz Sharif expects a reaction from the Taliban and has decided to call out the troops to guard sensitive areas in Islamabad from Aug 1.
Official sources said the troops have started taking positions in various parts of the capital city in aid of the police and civil administration, Xinhua reported.
They said nearly 2,000 troops will be deployed in key locations, mostly in the "Red Zone" where the president's the prime minister's houses are located as well parliament, the diplomatic enclave and some other important government buildings.
A spokesman for the interior ministry said the army troops will temporarily be deployed to aid the police.
The government has already deployed paramilitary troops in the capital city in support of the police force following a deadly attack on judges and lawyers in Islamabad earlier this year.
Major opposition parties have opposed the decision and fear that the army could be used to stop political gatherings. However, Interior Minister Chaudhry Nisar Khan has rejected the notion.
Khan said the troops would perform security duties in Islamabad for three months and that the constitution allowed troops' deployment in aid of the civil administration.
The interior minister said the troops would be on duty at Islamabad's Benazir Bhutto International Airport and other sensitive places in the capital city.
He said the army contingents from the Rapid Response Force will also be on duty to aid the police and civil administration.
The troops' deployment comes at a time when a major opposition party has announced an anti-government march in Islamabad Aug 14.
Cricketer-turned politician Imran Khan said hundreds of thousands of supporters of his Tehrik-e-Insaf or Justice Movement will march in Islamabad. Some other opposition parties said they would also join the march.
The government is bracing for the march on the day Pakistan celebrates its Independence Day.
Imran Khan said he would gather around one million people in Islamabad and the marchers will stage a sit-in unless the government accepts his demand for investigation into what he claimed fraud in the last year's parliamentary elections.