Nigeria’s petroleum regulator has revoked six oil block licenses due to “legacy debts,” it said in a public notice on June 6.

The notice, carried in some Nigerian newspapers, said the move by the Department of Petroleum Resources was “in furtherance of the presidential directive.”

The action comes as Nigeria takes a more aggressive approach to collecting taxes and royalties that the country says it is owed. Oil industry sources said Nigeria has also been increasingly vocal about rescinding licenses that are not being actively developed.

The notice said oil mining lease (OML) 98 was revoked from Pan Ocean Oil Corp., OML 120 and 121 from Allied Energy Resources Nigeria Ltd., OML 108 from Express Petroleum & Gas Co. Ltd. and OML 110 from Cavendish Petroleum Nigeria Ltd.

The notice also said oil prospecting license (OPL) 206 was revoked from Summit Oil International.

Details of the size or value of the blocks were not immediately available.

A spokesman for the president declined to comment. A petroleum ministry spokeswoman did not immediately respond to a request for comment.