KABUL - At least two rockets fired from undisclosed location hit the city center of Afghan capital Kabul on Monday, the day on which Taliban insurgent group said to launch their yearly offensive against security forces and government targets.

"The rockets landed at around 4:50 am local time. I cannot provide details on casualties. There is fear of casualties," a security source said.

On Thursday, the Taliban insurgents announced that they would launch their spring offensive.

"I heard two big bangs in central Kabul where presidential palace and majority of the foreign embassies are located. Following the blasts, siren was heard in the area. The US embassy alarm was also whistled shortly after the explosions," Shukrollah, resident of Makkarryan area said.

Several police vehicles were also running in the area, the witness said.

"The spring operation code named Khyber will be launched at 05: 00 am local time on Monday May 12, 2014 across Afghanistan, against invading forces with the objective to get the country's freedom from infields," said a statement sent by the Taliban on Thursday.

All foreign forces, their civilian and military contractors and anyone supporting foreign invasion is legitimate targets and will be attacked, the Taliban warned.

Ranking government officials, cabinet members, members of parliament, and security personnel, judges, prosecutors and members of security agency are our targets, the Taliban said.

In 2013, Taliban militants launched their offensive on May 28, which, according to Afghan security officials, had no achievement except little media propaganda.