United Nations diplomats reached agreement late Thursday on the wording of a draft Security Council resolution that would greatly strengthen a peacekeeping force in South Sudan, the world’s youngest country, which has been ravaged by civil war and suffering for nearly three years.

The resolution would basically give the United Nations far more authority in South Sudan, backed by lethal force if needed, to protect civilians and pressure armed antagonists in the conflict — including government soldiers.

It is expected to be approved on Friday, the expiration date for the mandate of the current peacekeeping operation, known as the United Nations Mission in South Sudan, or Unmiss.

A peace agreement has been repeatedly ignored, and the resolution, sponsored by the United States, represents an unusually robust action by the Security Council, invoking its rarely used coercive power to militarily intervene when international peace and security are considered to be threatened.

South Sudan’s government opposes the resolution, raising the possibility of clashes between the country’s armed forces and foreign soldiers deployed there by the United Nations.

Under the resolution, the United Nations’ mission would be extended and a new 4,000-soldier “regional protection force” would be deployed in South Sudan’s capital, Juba, and other strategic locations, including the airport.

The new force represents an increase of more than 30 percent in personnel for the United Nations mission, currently at 12,000, which has been unable to stop episodic bouts of killing and abuses, including widespread rape, by both government forces and rebel factions.

United Nations soldiers and aid workers have been repeatedly harassed and attacked, and in some cases killed. Thousands of South Sudanese civilians, fearing for their lives, have been living in United Nations facilities in Juba and other locations.

The resolution specifies that the new force, which diplomats said would mostly be drawn from neighboring countries, will be authorized to “promptly and effectively engage any actor that is credibly found to be preparing attacks, or engages in attacks, against United Nations protection of civilians sites, other United Nations premises, United Nations personnel, international and national humanitarian actors, or civilians.”

The resolution does not impose an arms embargo on South Sudan, as some international rights groups had wanted. But in what was created as a coercive step, the resolution allows for the imposition of an arms embargo if the government does not cooperate.

South Sudan’s promise as a newly independent state in 2011 devolved into civil war two years later, leaving tens of thousands dead and more than 2.3 million displaced.

Soldiers loyal to President Salva Kiir — who belongs to the Dinka ethnic group, South Sudan’s largest — battled troops led by Riek Machar of the Nuer ethnic group, which is believed to be the second largest.

Troops on both sides committed human rights abuses against civilians on a devastating scale.

A peace deal officially ended the fighting last year. Mr. Machar, who had served as vice president before he was dismissed in 2013, agreed to become Mr. Kiir’s deputy again and returned to Juba in April.

But then fighting broke out again between the two sides on July 7, killing hundreds. Mr. Machar’s residence was destroyed, and he fled the capital. He has refused to return to Juba unless more international troops are deployed.

Mr. Kiir has argued that the 12,000 troops from the United Nations mission already stationed in the country are sufficient.