The United States is alarmed by the deteriorating security in Sudan's Darfur region and wants the situation to be "effectively addressed," its ambassador to the UN said Tuesday.

"We are alarmed and gravely concerned," Susan Rice told reporters following a United Nations Security Council briefing on the situation in the western Sudanese region.

Commenting on a UN report describing May as the deadliest month -- with 400 civilians killed -- since the United Nations-African Union (UNAMID) force was set up in 2007, she said the deteriorating situation in Darfur "is unacceptable and needs to be effectively addressed."

She welcomed the fact that UNAMID is nearing full deployment, with over 17,000 troops and more than 4,000 police now on the ground.

But Rice stressed that Khartoum's "continued restrictions on UNAMID movements, (on) its ability to fly helicopters in defence of its mission including protection of civilians and evacuation of UNAMID peacekeepers under attack is absolutely unacceptable and needs to be halted."

She urged the Sudanese government to allow "full freedom of access and movements to UNAMID and for humanitarian workers." In his latest report to the 15-member Security Council, UN chief Ban Ki-moon appealed to Khartoum and the Justice and Equality Movement, Darfur's largest rebel group, "to immediately cease their ongoing military confrontation and commit to the peace process."

He said that JEM's withdrawal from peace talks in Doha "has undermined the goal of an inclusive and rapid resolution of the Darfur conflict."

Without such a deal in Darfur and with south Sudan likely to opt for independence in a referendum scheduled for January, "there is a risk of increasing instability in Sudan," Ban warned.

UNAMID chief Ibrahim Gambari for his part urged the Security Council to renew its appeal to all Darfur rebel groups to join the Doha peace talks "without any pre-conditions" to finalize a peace deal before the end of the year.

He also pressed the council to appeal to all parties in Sudan "to demonstrate restraint and refrain from actions likely to further complicate the security situation."

Darfur has been gripped by civil war since 2003 that has left 300,000 people dead and 2.7 million displaced, according to the United Nations. Khartoum says 10,000 have been killed in the conflict.