SYDNEY, March 5 (Xinhua) -- A 6.0-magnitude earthquake has struck 111 km southwest of Porgera in Papua New Guinea on Monday, according to the U.S. Geological Survey.

The tremor has followed on from a series of large aftershocks which have ravaged the Southern Highlands region after a powerful 7.5-magnitude quake hit the area on Feb. 25.

The latest tremor, which occurred at 1956 GMT Sunday, was initially determined to be at 6.307 degrees south latitude and 142.620 degrees east longitude at a depth of 10 km.

According to the Red Cross, although the area has no major urban centers, around 670,000 people live within 100 km of the epicenter.

"We haven't slept. It's been shaking all through the night," said provincial administrator of Hela Province William Bando, who was 40 km away from the epicenter.

"What we experienced this morning could have caused more damage, but we don't know ... it almost threw me out of bed."

Latest reports estimated that at least 31 people have been killed since the in initial tremor and aid agencies said that 150,000 people still remain in urgent need of supplies near the townships of Mendi and Tari.

Emergency deliveries have been hampered due to landslides, damaged roads, collapsed buildings and the remote location of the area.

Papua New Guinea Prime Minister Peter O'Neill has declared a state of emergency.