Afghanistan's President Hamid Karzai has blamed NATO intelligence failures for a series of coordinated attacks throughout the country on Sunday.

Karzai went on to praise Afghan forces for handling the situation, saying they had proven themselves capable of defending their country.

On Sunday, over 38 militants and eight Afghan security forces were reportedly killed after an 18-hour battle with insurgents in the capital Kabul and in other provinces across the country.

The terrorists' infiltration in Kabul and other provinces is an intelligence failure for us and especially for NATO and should be seriously investigated, Karzai said, according to Agence-France Presse.

According to experts, the attacks may signify the start of the Taliban's spring fighting season.

Foreign embassies, NATO headquarters and the Afghan parliament were hit by a series of attacks initiated by either Taliban insurgents or Haqqani fighters Sunday, the first major assault on an Afghan city in more than six months, and one of the most serious since the US-backed Afghan forces removed the Taliban from power in 2001.

Dozens of militants armed with machine guns, rocket-propelled grenades and suicide vests also launched further attacks in Nangarhar, Logar and Paktia provinces.

Loud explosions and intense firefighting erupted in the early hours of Monday as security forces tried to flush out the militants holed up in the diplomatic enclaves of Kabul.

The Taliban claimed responsibility for the attacks, but some officials said the Haqqanis, a network of tribal militants linked to Al-Qaeda who live along the Pakistan-Afghanistan border, were likely involved.

My guess, based on previous experience here, is this is a set of Haqqani network operations out of north Waziristan and the Pakistani tribal areas, U.S. Ambassador Ryan Crocker told CNN. Frankly I don't think the Taliban is good enough.

CNN added that the sole surviving militant also claimed to be working for the Haqqanis.