Afghanistan's capital city is under tighter security after brazen attacks from the Taliban.

At least 15 people have been killed and nearly 40 wounded in suicide bombings in Afghanistan's capital city of Kabul.

Taliban militants launched a series of attacks Monday in the heart of Kabul, including a suicide bombing near the presidential palace.

There were also fierce gun battles in the streets between insurgents and Afghan police.

The Afghan government is questioning how the Taliban got through so many security checkpoints.

The attacks were aimed at key government targets in a clear sign the insurgents plan to escalate their fight. The insurgents have become increasingly brazen in challenging Afghan and international forces as the U.S. ramps up its campaign to end the war.

A statement on a Taliban Web site said insurgents had entered the city specifically to target the presidential palace, the Central Bank, the Ministry of Mines and Industry and the Serena.

Mohib Safi, deputy governor of the central bank, said bank employees heard a strong explosion followed by gunfire. He said employees were safely inside and no militants had entered the building.

A reporter with The Associated Press said Afghan security forces surrounded the area, and that fighting between the two sides was continuing. Ambulances transported the wounded from the scene.

NATO said the explosion was reported near the Central Bank and that international forces were working with Afghan troops to secure the area.