Kabul attack highlights security challenges in Afghanistan as US mulls more troops The Pentagon proposed 2,000-4,000 more troops for Afghanistan.

 -- At least 90 people have been killed and some 400 others injured in a huge car bomb explosion in Kabul, Afghanistan targeting the city’s diplomatic neighborhood that is also close to the presidential palace.

At least 11 American contractors working for the U.S. embassy are among the injured, according to a U.S. official. The attack highlights the continuing security threat posed by insurgent groups in Afghanistan.

There has been no claim of responsibility for the blast in the city district known as Wazir Akbar Khan that is known as one of the securest parts of the capital.

The Afghan intelligence service NDS said in a statement that their initial findings pointed to the Haqqani Network as being responsible for the attack. The Haqqani Network is an extremist group that operates mainly in northeast Afghanistan and that has at times been aligned with the Taliban and received support from Pakistan's intelligence agency.

In recent years the Taliban has become resurgent throughout Afghanistan as Afghan forces took the lead for security in the country and American and coalition forces stepped back into an advisory role.

But the security challenges in Afghanistan have grown beyond fighting the Taliban and the Haqqani Network. In late 2015, warring groups formerly aligned with the Pakistani Taliban switched their allegiance to ISIS and re-branded themselves as ISIS-Khorasan and quickly became the primary security challenge in eastern Afghanistan.

With increased U.S. military support, Afghan forces have turned a corner against ISIS, dealing the group significant battlefield losses over the past year.

Meanwhile, the Taliban has expanded its operations challenging Afghan forces on several fronts. Over the past year they have maintained pressure in northern Afghanistan seeking to retake the city Kunduz that they briefly seized in late 2015.

Earlier this year, U.S. General John Nicholson described the current military situation in Afghanistan against the Taliban as a "stalemate".

While the results on the battlefield have been mixed, high profile terror attacks have continued to hit the relatively stable capital city of Kabul.

American military officials believe the insurgent groups carry out attacks in Kabul to demonstrate the weakness of the U.S-backed Afghan government and its security forces.

The Trump administration is currently conducting a review of the U.S. strategy for Afghanistan. Secretary of State Rex Tillerson told reporters last week that the Trump administration is weeks away from completing the review. As part of that process the administration will consider whether additional American troops should be sent to Afghanistan.

General Nicholson has said he needs additional U.S. and coalition forces to enhance the 8,400 American and 5,000 coalition forces advising and training Afghan security forces.

The Pentagon has proposed that between 2,000 and 4,000 additional American troops might be needed for Afghanistan, with a similar troop commitment from other NATO countries, according to a U.S. official. The number of additional troops would depend on the troop level commitments made by NATO countries.

ABC News' Aleem Agha contributed to this report.