(Adds U.S., British, South Sudan government and opposition comments)

By Michelle Nichols

UNITED NATIONS Aug 12 (Reuters) - The United Nations Security Council on Friday authorized the deployment of a 4,000-strong protection force in South Sudan's capital, Juba, as part of the U.N. peacekeeping mission and threatened an arms embargo if the government does not cooperate.

The U.S.-drafted resolution was adopted with 11 votes in favor. Russia, China, Egypt and Venezuela abstained.

The authorization follows several days of heavy fighting involving tanks and helicopters in Juba last month between troops loyal to President Salva Kiir and those backing former Vice President Riek Machar that raised fears of a return to full-scale civil war in the world's newest nation.

"Until the leaders of South Sudan are willing to put what is good for their people before themselves, putting peace ahead of personal ambition and power ... the people of South Sudan will continue to suffer from the bloodshed and instability their leaders reap," U.S. Deputy U.N. Ambassador David Pressman said.

The protection force of African troops will use all means to enforce peace in Juba and protect the airport and other key facilities, the Council said. It would also act against anyone who is "preparing attacks, or engages in attacks" against U.N. sites, aid workers or civilians and could confront South Sudanese government troops if needed.

Hundreds of people were killed and the United Nations said government soldiers and security forces executed civilians and gang-raped women and girls during and after last month's fighting. South Sudan rejected the accusations.

The protection force will be part of the U.N. peacekeeping mission in South Sudan, known as UNMISS, in place since the country gained independence from Sudan in 2011. The force's chief will report to the UNMISS commander.

The council authorized UNMISS and the protection force until Dec. 15, 2016.

East African bloc IGAD pushed for the creation of the Juba protection force. South Sudan said it would accept a deployment of African troops in the capital, but on Wednesday voiced opposition to those troops being placed under U.N. command.

After the Security Council vote on Friday, a spokesman for Kiir said the government would not accept the force.

"We are not going to 'cooperate' on that because we will not allow our country to be taken over by U.N.," Ateny Wek Ateny told Reuters. "Any force that will be called Juba Protection force will not be accepted."

Under Friday's resolution, the council will consider imposing an embargo if U.N. Secretary-General Ban Ki-moon reports that South Sudan's government is obstructing deployment of the protection force or the work of UNMISS.

Several council members wanted an immediate arms embargo.

"Today we also had a chance to stop the violence by implementing an immediate arms embargo on South Sudan. On that we have failed," Britain's deputy U.N. ambassador, Peter Wilson, told the Council.

The deployment of more international troops in Juba has been a key demand of Machar, who left Juba following the violence last month. Kiir has since appointed a new vice president.

A spokesman for Machar welcomed the proposed deployment of the U.N. protection force in Juba.

South Sudan descended into civil war after Kiir first dismissed Machar as his deputy. They signed a peace deal in August 2015, but implementation was slow and difficult.

(Additional reporting by Denis Dumo in Juba; editing by Dan Grebler)