GENEVA (Reuters) - The United Nations human rights boss Michelle Bachelet called on Friday for an independent investigation into alleged excessive use of force by Venezuelan security forces or allied militia, citing reports of at least 20 people killed this week.

More than 350 people are reported to have been detained in protests since opposition leader Juan Guaido declared himself interim president, challenging President Nicolas Maduro, she said.

“I am extremely concerned that the situation in Venezuela may rapidly spiral out of control with catastrophic consequences”, Bachelet, a former president of Chile, said in a statement issued in Geneva.

“Any violent incident resulting in death or injury should be subject to an independent and impartial investigation to find out whether there was excessive use of force by the authorities, or if crimes have been committed by members of armed groups, whether pro-government or otherwise,” she said.

Bachelet urged Venezuela’s political leaders to hold talks to defuse the “increasingly incendiary atmosphere” brought on by an entrenched social, political and economic crisis.

The United States is seeking to ensure that Venezuelan oil revenue goes to Guaido, president of the National Assembly, and to cut off money from isolated Maduro, a U.S. official said on Thursday.

The U.N. refugee agency UNHCR said it was monitoring the situation but had not yet seen an increased outflow of Venezuelans, more than 3 million of whom have already fled the crisis. The United Nations warned last month that an estimated 2 million Venezuelans could join their ranks in 2019.

“We are still preparing for any potential change in the number of refugees and migrants leaving Venezuela,” UNHCR spokeswoman Liz Throssell told a news briefing.

“What we have seen is that population movements so far have remained constant since last year, and that is some 5,000 Venezuelans leaving their country every day on average.”

Many transit Colombia on their way to Peru or Ecuador, she said, adding: “We have seen a significant number in dire need of international refugee protection and humanitarian assistance.”