The head of Kremlin-controlled energy giant Rosneft targeted the Organization of the Petroleum Exporting Countries (Opec) and the US as he launched a broadside at the forces behind the oil price crash that has hit Russia’s economy hard.

Igor Sechin, one of President Vladimir Putin’s closest allies, said during a rare visit to London that Opec had erred in not cutting output, as he blamed the low oil price on factors from financial speculators to US government policy. Oil has more than halved since June as fast-growing US shale supplies overwhelmed demand. The decline accelerated after the Opec, of which Russia is not a member, chose to try protect its market share rather than cutting output to support prices.

“Opec has lost its teeth,” Sechin said through an official translator at London’s International Petroleum Week, an annual industry event. Opec’s decision to let prices fall had led to a “destabilisation” of the market, he said. Sechin, who has been targeted by Western sanctions over Russia’s role in the Ukraine crisis, said the sharp collapse in oil could not be explained only by the forces of supply and demand, hinting there may be other factors at play.

While careful not to go as far as some politicians and analysts, who have suggested the price crash was orchestrated by the US and Saudi Arabia to hurt Russia and Iran, Sechin nevertheless said prices have been “manipulated”.