The United States has taken "extraordinary security precautions" to protect U.S. diplomats in Khartoum following the infiltration of several hundred Libyan agents into the Sudanese capital, U.S. officials said yesterday.

The officials refused to detail the precautions, but said there has been "a lot of concern" about security since Sudan's powerful State Security Organization was dismantled after the April 6 military coup that toppled President Jaafar Nimeri.

"Part of that concern is the security of our embassy," one official said.

The officials said the new military leadership under Abdel Rahman Sawar-Dhahab has told Washington it is no longer able to keep track of all the Libyans and their Sudanese allies, leaving U.S. diplomats vulnerable.

The U.S. government has been particularly sensitive about security because of the March 1973 seizure and murder of then-U.S. Ambassador Cleo A. Noel Jr. and his deputy, G. Curtis Moore, by eight Palestinian terrorists.

Since the April 6 coup, "over 100 and maybe as many as a couple of hundred" Libyans have arrived in Khartoum with the reestablishment of diplomatic relations between the Sudan and Libya. They have been busy setting up "revolutionary committees" to promote a Libyan-style jamahiriya, or "state of the masses."

These committees have been used in other countries, such as Egypt, to carry out subversive activities.

In Britain, four Libyan students seized control of their country's embassy in February 1984 and declared themselves a revolutionary committee that had displaced the ambassador. Two months later someone in the embassy building shot and killed a British policewoman, leading to a break in British-Libyan relations.

Also returning in large numbers have been Libyan-trained Sudanese, among them a man named Zakaria, regarded as especially dangerous. Described as a "macho guerrilla type," he arrived with 100 followers about two months ago after several years in exile; some in the group carried Soviet-made AK47 rifles.

In some cases, U.S. officials said, a plane has arrived from Libya with 100 people on it but only 80 passports, the others slipping through the relaxed security at the airport.

Officials describe the political situation in Khartoum as "highly fluid" with many groups, including communists, Baathists and Libyan-backed elements, jockeying for power. The current military leadership has promised to hold elections for a new parliament and civilian government by April 6, 1986.

Libyan leader Muammar Qaddafi visited Khartoum briefly May 18. But his number two aide, Abdul Salaam Jalloud, was there previously on a week-long visit, after which many of those who accompanied him stayed on, according to U.S. intelligence reports.

Qaddafi has promised "all kinds of aid" to the new Sudanese government, including badly needed food for famine victims and oil, but it is unclear how much has arrived, U.S. officials said.