The cost of keeping watch on Julian Assange, who is holed up in the Ecuadorean embassy in London, is proving a drain on police resources and the operation is under review, London's police chief said on Tuesday.

The WikiLeaks founder sought refuge in the embassy in June 2012 to avoid extradition to Sweden, where he is wanted for questioning over allegations of sexual assault, which he denies.

He faces arrest for breaching British bail conditions if he leaves the embassy, which is in the upmarket central London neighbourhood of Knightsbridge.

London's Metropolitan Police have been standing guard outside the embassy around the clock to prevent him from fleeing, at an estimated cost of about 10 million pounds ($15 million) since the operation began.

"We are reviewing ... how we can do that differently in the future because it's sucking our resources in," Metropolitan Police Commissioner Bernard Hogan-Howe told LBC radio, adding that the force was facing funding cuts.

"We won't talk explicitly about our tactics, but we're reviewing what options we have," he said, declining to be drawn on whether fewer police might be deployed outside the embassy.

Assange's presence inside the embassy has caused a diplomatic impasse between Britain and Ecuador, which has granted him asylum.

London says that having tried and failed to challenge Sweden's extradition request through the courts, Assange must obey the law and hand himself over to be flown to Stockholm.

Assange says that the allegations against him were trumped up to facilitate his extradition first to Sweden but ultimately to the United States, where he could be put on trial over huge leaks of previously secret information to WikiLeaks.

The website began publishing thousands of U.S. diplomatic cables in 2010, infuriating Washington and, according to Assange's critics, damaging U.S. national security and putting the lives of some dissidents in authoritarian countries at risk.