An explosion and gunfire rattled Kabul's diplomatic district on Wednesday as insurgents dressed as doctors attacked Afghanistan's largest military hospital, officials said, as growing insecurity besets the war-battered country.

No insurgent group immediately claimed responsibility for the ongoing assault on the Sardar Daud Khan hospital, but it comes as the Taliban ramp up attacks even before the start of their annual spring offensive.

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No casualties were immediately reported but medical staff trapped in the facility posted desperate messages for help on social media. "Attackers are inside the hospital. Pray for us," a hospital staff member wrote on Facebook.

Hospital administrators told AFP three gunmen wearing white laboratory coats were on the loose after the explosion struck, sparking chaos inside the 400-bed facility.

"The attackers are shooting everywhere," administrator Abdul Hakim told AFP by telephone.

"We are trying to bring the situation under control," he said in a frantic hurry before hanging up.

The attack comes a week after 16 people were killed in simultaneous Taliban suicide assaults on two security compounds in Kabul.

Dozens of others were wounded as a suicide car bomber struck an Afghan police precinct in western Kabul and a five-hour gun battle ensued after another attacker snuck in, sending clouds of smoke billowing into the sky.

In the second attack, a suicide bomber blew himself up at the gates of an Afghan intelligence agency branch in eastern Kabul.

The growing violence underscores rising insecurity in Afghanistan over the resurgent Taliban.

The country is bracing for an intense fighting season in the spring as the government's repeated bids to launch peace negotiations with the Taliban have failed.

Afghan forces, already beset by record casualties, desertions and non-existent "ghost soldiers" on the payrolls, have been struggling to rein in the Taliban since US-led NATO troops ended their combat mission in December 2014.

Kabul last month endorsed US general John Nicholson's call for thousands of additional coalition troops in Afghanistan to fend off the militants before the spring offensive.

Extra troops were needed to end the stalemate in the war, Nicholson, the top US commander in Afghanistan, told the US Congress in what could be President Donald Trump's first major test of military strategy.

Separately, the Pentagon this year said it would deploy some 300 US Marines this spring to Helmand province alone.