Royal Dutch Shell has bought $1.5 billion worth of crude oil from the National Iranian Oil Company (NOIC), Britain's Guardian newspaper said, citing sensitive trading documents.

The newspaper said the UK-registered company this summer stepped up its orders of Iranian oil while other major buyers -- including BP and India's Reliance Industries -- halted orders following U.N.-imposed sanctions in June in reaction to Iran's uranium enrichment program. Wide-ranging international sanctions implemented in June and July have excluded crude oil sales, but Western-allied powers have increased political pressure on countries to trade less with Iran.

"We continue to comply with all legislation," the Guardian quoted a Shell spokesman as saying, adding that the company does not comment on its trading activities. The paper quoted a Foreign Office spokesman as saying Britain "will rigorously enforce sanctions."

No immediate comment was available from the Foreign Office, while Shell was not immediately reachable.