Smoke and flames are seen at Royal Dutch Shell's Pulau Bukom offshore petroleum complex in Singapore September 29, 2011. [Photo/Agencies]

SINGAPORE - Firefighters continued to work hard at a plant of Shell in Singapore on Thursday as the fire continued into a second day.

Witnesses at the southwest coast of Singapore said they heard explosions around noon on Thursday and saw more black smoke and fireballs shooting into the sky from the refinery located on Pulau Bukom, a small island of 1.45 square kilometers that lies five kilometers off the coast of the main island.

The fire broke out on Wednesday afternoon, involving petroleum products from pipes in the tank farm at the manufacturing facility measuring 150 metres by 50 metres.

Local residents woke up on Thursday to a smell of smoke in the air. But authorities said the smell and haze were not due to the fire in Singapore but due to farmers burning the weeds in their land in Indonesia.

In an update on Thursday, the Singapore Civil Defence Force (SCDF) said there has been a surge in the fire at the Shell manufacturing facility.

Local broadcaster Channel NewsAsia said the fire is still contained within the bund wall.

The SCDF said about 20 water jets are being used to carry out boundary cooling operations to prevent heat exposure to the nearby storage tanks.

Two fire engines were badly damaged and another one sustained partial damage, but so far there were no injuries.

Shell has evacuated all non-essential staff from Pulau Bukom from the island, but about 250 essential personnel of the company remained on the island to help with the operations.

"We believe it was an accident. A full investigation will be conducted once the fire is put out," Shell said in a statement.

The refineries in Singapore, a major refining center, are concentrated on and near the Jurong Island in the southwest.