Image copyright AFP Image caption Security forces have been exchanging gunfire with a group of suspected militants holed up in an apartment building in Sylhet

Six people - two police officers and four civilians - have been killed in twin bombings in north-eastern Bangladesh, officials say.

The blasts in Sylhet occurred near an apartment building where commandoes have been trying to flush out a group of suspected Islamist militants.

Earlier many civilians were evacuated from the block of flats. The suspects have refused to surrender.

Dozens of people were injured in the twin explosions on Saturday.

Army units continue to surround the building.

The blasts hit a large crowd which had gathered nearby as troops continued to exchange gunfire with the militants.

The first explosive device was brought by two men on a motorbike and the second was left in a bag of vegetables, police say.

So-called Islamic State (IS) claimed on messaging app Telegram that it was behind one of the blasts.

The suspected extremists are hiding out in a five-storey building, from which 78 civilians have been evacuated.

They keep detonating explosives strewn all around the building, the army says.

Police began their siege on Friday morning, the same day that an apparent suicide attack took place near the main airport in Dhaka, the capital.

Only the bomber died, and IS said it was behind the attack. There was another suspected suicide bombing in the Dhaka area on 17 March, which targeted the barracks of an elite police force, injuring two personnel.

A day later, a man was shot dead at a checkpoint of the same force, and police said bombs were found on him.

A violent resurgence - Analysis by Anbarasan Ethirajan, BBC South Asia Analyst

The escalation in violence comes at a time when many in Bangladesh thought security forces had managed to curb Islamist militancy, following the deadly siege on a café in Dhaka last year.

Since then security personnel have carried out a number of raids, arresting and sometimes killing dozens of suspected Islamists across the country.

But the recent suicide attacks on security camps and checkpoints have surprised many.

Despite claims by the Islamic State group and al-Qaeda, Bangladeshi police maintain that an offshoot of the banned Islamist outfit Jamiatul Mujahideen Bangladesh (JMB) is responsible for many of these attacks.

The challenge for the security forces is to find out how many more followers or hard line jihadist sympathisers are still in hiding.

If it is proved that Islamist extremists are behind the recent handful of suicide bombings, then it marks a new phase in Bangladesh's fight against militancy.

The rise in violence may also trigger concerns among secular activists, religious minorities, academics and bloggers who bore the brunt of the jihadist violence in the past few years.