Central Asia correspondent, BBC News
Russia is trying to boost its presence in Central Asia
Tajikistan is hosting a regional security summit amid fears the conflicts in Afghanistan and Pakistan may be spreading into Central Asia.
The presidents of Pakistan, Russia, Afghanistan and Tajikistan are to meet in the Tajik capital Dushanbe.
Clashes between militants and police have led to concerns that unrest in countries on Tajikistan's southern border could be spreading there.
The Tajik government says the militants have links to the Taleban.
On the eve of Thursday's summit, Pakistan's President Asif Ali Zardari said his country would stand together with Tajikistan to fight extremism.
"It threatens my brother's country, it threatens my country, and it threatens the neighbourhood," said Mr Zardari, after meeting Tajikistan's President Emomali Rakhmon in Dushanbe.
In recent weeks there have been a number of clashes between armed militants and Tajik security forces in the eastern Rasht Valley.
On Wednesday, Tajik officials said they had killed a senior militant who belonged to a Central Asian group called the Islamic Movement of Uzbekistan.
Rasht is a remote area close to the Afghan border, and observers have speculated that the violence in Tajikistan may be related to an intensified campaign by Pakistan's military against the Taleban and its supporters in the Afghan-Pakistan border area.
Tajikistan's President Emomali Rakhmon is hosting Mr Zardari, President Hamid Karzai of Afghanistan and Russian President Dmitry Medvedev.
The four are expected to discuss security issues as well as a future energy project that will link Afghanistan and Pakistan to a proposed hydroelectric power grid supplied by dams in Tajikistan and Kyrgyzstan.