Severe tropical storm ''Sarah'' intensified as it moved northwest over the Philippine Sea, the Philippine Atmospheric, Geophysical and Astronomical Services Administration ( Pagasa) said Thursday.

From maximum sustained winds of 100 kilometers per hour and gusts of 125 kilometers per hour, by 5 pm ''Sarah'' strengthened with winds of 110 kilometers per hour and gusts of up to 135 kilometers per hour.

Pagasa said the center of ''Sarah'' was at 255 kilometers east of Basco, Batanes. It is moving northwest at 10 kilometers per hour.

Tropical Cyclone Wind Signal Number One was raised over Batanes.

"For the following hours,light to moderate rains over Isabela, Cagayan (especially on the eastern section), and northern Aurora due to the trough of Severe Tropical Storm "Sarah," said Pagasa.

The weather bureau announced sea travel is risky over areas under tropical cyclone wind signal, seaboards of Northern Luzon, and western seaboard of Central and Southern Luzon due to rough seas caused by ''Sarah'' and the northeast monsoon.

''Sarah'' is expected to weaken on Friday or Saturday with Pagasa forecasting it will leave the Philippine Area of Responsibility Saturday morning. Cristina Eloisa Baclig/DMS