US benchmark contract West Texas Intermediate for June delivery ended at $70.73 per barrel, up $1.01.

European benchmark Brent North Sea crude for July delivery gained $1.30 to $76.17, also its highest level since late 2014.

British Foreign Secretary Boris Johnson was the latest European official to travel to Washington to urge the Trump administration to preserve the 2015 pact in which major powers agreed to ease some sanctions on Iran in exchange for greater scrutiny of its nuclear activities.

US President Donald Trump tweeted that he would announce his decision Tuesday.

Trump has until May 12 to certify that Iran is complying with the agreement curbing its nuclear program, or clear the way for resumed US economic sanctions.

"The market is worried that President Trump will not certify that Iran is compliant and as a result, that may reduce the amount of Iranian exports at a time when oil inventories are declining," said Andy Lipow of Lipow Oil Associates.

Oil traders are also nervous over the prospects of more aggressive US policies against Venezuela, another major oil exporter.

US Vice President Mike Pence urged Venezuelan leader Nicolas Maduro to suspend a May 20 election boycotted by the opposition, denouncing the vote as a "fraud."

Pence's remarks came as the US Treasury Department imposed new sanctions on three Venezuelans, including a former senior intelligence official over narcotics trafficking.

The US also named 20 companies in the action, including 16 in Venezuela.