MOSCOW (AP) — Major oil producers need to show greater discipline in sticking to output cuts aimed at raising the price of crude, Russia's energy minister said Monday.

OPEC and several non-OPEC states like Russia decided last year to rein in output, but prices have fallen below $50 a barrel in recent weeks amid concerns about higher production in the U.S., which was not part of the deal, and some countries' lack of discipline in enforcing the cuts.

Following a meeting of OPEC and non-OPEC countries in the Russian city of St. Petersburg on Monday, Alexander Novak said complained that "some countries are not yet fully implementing" the cuts.

"Despite the high level of compliance with the agreement, we insist on all countries fulfilling their obligations 100 percent," he said, in comments reported by Russian state news agencies.

Novak added that he would be open to tighter monitoring of output, and a possible extension of the cuts beyond their scheduled end in March 2018.

The International Energy Agency has estimated that compliance with the OPEC output cut fell to 78 percent in June, from 95 percent the previous month.

Beyond high U.S. output, oil prices have also been pressured by production increases in OPEC members Libya and Nigeria, which have exemptions from the cuts due to political instability.