A strong 6.1-magnitude earthquake struck off the Pacific nation of Tonga Monday, US seismologists said, but no tsunami warning was issued. The US Geological Survey (USGS) said the quake was shallow, striking 154 kilometres (95 miles) north-west of the village of Pangai, the administrative capital of the Ha'apai group of islands in central Tonga. The region's homes are vulnerable to earthquake shaking, according to USGS, but it said there was a low likelihood of casualties and damage.