Nematullah Kundi, a Pakistani police official, said: "This is a high alert. We have stepped up security in the city, in and around the diplomatic enclave and the area near the parliament building, which is the declared red zone.
"Extra guards have also been deployed at schools in the red zone and elsewhere in the city."
While much of the recent violence has been concentrated in northwest Pakistan, where the army has been fighting pro-Taliban fighters, many fear Islamabad could be next.
Taliban 'strengthening'
Mullah Nazeer Ahmed, a senior Pakistani Taliban commander, said: "The mujahidin are getting stronger by the day and ... If they [the US military] continue to attack us, then our soldiers will reach Islamabad."
Ahmed is based in the province of South Waziristan, where a suspected US missile attack last week killed at least three fighters.
Scores of Pakistani Taliban fighters have already moved from their stronghold in the Swat valley into Buner, a district located northwest of Islamabad.
Israr Bacha, a local police officer, said: "About 20 vehicles carrying Taliban entered Buner on Monday and started moving around the bazaar and streets."
Talat Masood, a senior Pakistani security analyst, told Al Jazeera that the fighters "can easily sneak into the capital".
"It is very difficult to control all entry points and these militants are very determined," he said.
Drone attacks
As the security situation in Afghanistan has worsened, the number of suspected US drone attacks across the border in Pakistan has increased.
Baitullah Mehsud, the Pakistani Taliban chief who has a $5m US bounty on his head, has cited such attacks as the reasons for its recent assaults on Pakistani security targets, including a police academy in Lahore.
He has vowed to stage at least two attacks every week as long as the US missile attacks continue.
"Every time there is a threat issued either by Mehsud or his colleagues, the government puts the city on high alert," Imtiaz Gul, an Islamabad-based political analyst, told Al Jazeera.
"Pakistan and the US are losing the war against terrorism, and that is the objective of al-Qaeda and the Taliban.
"The way the government responds to these security threat could be interpreted as a success for the Taliban."