By Michelle Nichols

JUBA, Sept 4 (Reuters) - The United Nations Security Council, acting on concerns South Sudan could again fall back into full-scale civil war, arrived in the country on Friday to demand that President Salva Kiir's government stop obstructing U.N. peacekeepers and cooperate on the deployment of 4,000 more foreign troops or possibly face an arms embargo.

The 15-member council agreed last month to consider an arms embargo on South Sudan, the world's newest nation, if U.N. Secretary-General Ban Ki-moon reports next Thursday that Kiir's government has not stepped up its efforts to work with the world body.

"It would be premature to assess whether the level of cooperation is sufficient, but ... it is extremely important for us to convey to the government of South Sudan that time is of the essence," U.S. Ambassador Samantha Power told reporters.

The council is due to meet with Kiir and his ministers over the weekend.

"The international community is extremely frustrated with the obstruction of U.N. peacekeeping operations that has gone on for too long," said Power, who, along with Senegal, is co-leading the three-day Security Council visit.

Fierce fighting in mid-July between troops loyal to Kiir and those backing former Vice President Riek Machar raised fears the five-year old state could slide back into full civil war, prompting the Security Council to approve a regional protection force to ensure peace in Juba.

East African regional trade bloc IGAD pushed for the protection force, which will fall under the command of the U.N peacekeeping force known as UNMISS.

"This regional protection force can be very important in enhancing the sense of security and building confidence and in allowing UNMISS to have capacity to go out and about and go beyond the protection of civilian sites," Power said.

Since civil war erupted in South Sudan in December 2013, sparked by a longtime political rivalry between Kiir and Machar, U.N. peacekeepers have been protecting tens of thousands of civilians sheltering at several U.N. bases around the country. Thousands of people have been killed and millions displaced by the conflict.

Kiir and Machar signed a peace deal a year ago, but fighting has persisted and Machar has now fled the country and is in Sudan.

"We should send a very strong signal that there is unity in the council that the current state of a affairs is a no-go," said Russian Deputy U.N. Ambassador Petr Iliichev.

A senior council diplomat, speaking on condition of anonymity ahead of the trip, said the levels of cooperation needed from the South Sudan to avoid an arms embargo are "nowhere near being met."

Iliichev said, however, that Russia - a council veto power - was not comfortable with the idea of imposing an arms embargo.

"The main problem is not with new weapons coming in. The country is inundated with weapons and nothing is being done on disarmament, demobilization and reintegration," he said.

The council will also assess the "ability and willingness" of peacekeepers to protect civilians and aid workers in danger, according to the terms for the trip, seen by Reuters, following accusations that U.N. troops failed to respond properly to deadly attacks and rapes, most recently in Juba.

The United Nations is investigating the accusations.

"We have a lot of questions about how those attacks can have occurred and why there has been no visible accountability for the perpetrators of those attacks," Power said. (Editing by Steve Orlofsky)