Bangkok: Authorities in Thailand tightened security in Bangkok as they probed a series of small blasts that hit the city on Friday morning and injured at least four people.

Six bomb blasts hit three sites, as the Thai capital hosted a south-east Asian security meeting with top diplomats from Australia, the United States, China and other world powers.

Thai investigators examine a site of an explosion that injured people in Bangkok on Friday. Credit:AP

The first small explosions were heard during rush hour, just before 9am local time at two sites near central Bangkok. A third blast was heard shortly afterwards at a government complex hosting several ministries on the northern side of the city.

Four people were wounded, the Erawan Medical Centre said. It added that "none are serious and all are receiving medical attention at hospitals."