Four powerful explosions - three in Dibrugarh and one in Charaideo districts - rocked Upper Assam on Sunday morning, as the country celebrated its 71st Republic Day.

All four blasts occured in a span of 10 minutes - between 8.15 and 8.25 am, a police officer said.

There has been no report of any casualty, as Republic Day being a holiday, people were mostly indoors, he said.

The first blast was reported outside a shop at Teokghat in Sonari police station area of Charaideo district.

Three explosions followed soon after in Dibrugarh district - two at Graham Bazar and AT Road, beside a gurudwara, and another at the oil town of Duliajan Tiniali, just 100 metres from the local police station, Additional Superintendent of Police (ASP) Padmanabh Barua told PTI.

CCTV footages available at Duliajan Tiniali showed two motorcycle-borne youths lobbed a grenade and sped away, the ASP explained.

The blasts at Graham Bazar and AT Road were carried out with time-induced Improvised Explosive Devices, he stated.

Senior police officers have rushed to the site of the explosions and a probe has been initiated, Barua added.

According to sources, the explosions were suspected to be the handiwork of the ULFA(Independent), but nothing has been established as yet.

The ULFA(I), along with several other proscribed outfits in the Northeast, had called for a boycott of the Republic Day celebrations.

Taking to Twitter, Assam Chief Minister Sarbananda Sonowal condemned the blasts and vowed to take strong action against the culprits.

"Strongly condemn the bomb blasts in a few places of Assam. This cowardly attempt to create terror on a sacred day only exhibits the frustration of the terror groups after their total rejection by the people. Our Govt will take the sternest action to bring the culprits to book," he tweeted.

A source in the chief minister's office said Sonowal has asked Assam Director General of Police Bhaskar Jyoti Mahanta to deal with the situation and nab the culprits involved in the explosions at the earliest.