Somali pirates have seized two European-operated tankers far out in the Indian Ocean in the last two days, in an apparent sign of resurgent activity by the group that seized the huge crude oil carrier Sirius Star last year.

The Nipayia, a Greek-operated chemical and oil tanker with 19 crew, was seized 300 nautical miles off Hobyo, in Somalia, according to Nato’s piracy information service, while the Norwegian-operated Bow Asir, with 27 crew, was seized 250 miles off the southern Somali town of ­Kismayo.