The Soviet-installed Afghan government has for the first time provided a specific timetable for the withdrawal of Soviet military forces in connection with a negotiated settlement of the Afghan war, U.S. and diplomatic sources said yesterday.

The Pakistani government, according to the sources, has rejected the proposed timetable as too lengthy. At the same time, though, the sources said Pakistan considers the proposal to be the basis for further negotiation and has agreed to a new round of United Nations-sponsored indirect talks with Afghanistan in early May.

State Department officials said details of the proposed Soviet withdrawal plan, including its length, are being kept secret by a few top officials of the government of Pakistan and by U.N. mediator Diego Cordovez, who received the plan from the Afghan government in the middle of last month and took it immediately to Pakistan. U.S. officials hope to learn more when Cordovez meets soon with Undersecretary of State Michael H. Armacost.

Informed sources said the Afghan government plan, which is presumed to have been drawn up in Moscow, calls for the phased withdrawal of all 118,000 Soviet troops in return for a halt to outside support for the Afghan rebels. The plan is reported to call for monitoring by impartial observers to determine that outside support for the rebels has ended, which would be required before Soviet troop withdrawal begins. Pakistani President Mohammed Zia ul-Haq said in an interview published last Thursday by the French newspaper Le Monde that Moscow has given "very positive signs" of readiness to negotiate a withdrawal of its forces. Asked if he thought Soviet leader Mikhail Gorbachev might review Soviet policy toward Afghanistan and consider the Soviet invasion a mistake, Zia said, "It is very possible that Mr. Gorbachev has changed his mind."

The withdrawal plan given to the U.N. mediator is viewed skeptically in Washington, where a senior official suggested it might be part of a public relations ploy directed from Moscow while the fighting in Afghanistan continues or even intensifies.

State Department officials said Soviet troops appear to be preparing for a major offensive against rebel bases along the Pakistani border in eastern Afghanistan. The officials said the heaviest fighting of this year has been taking place for more than a week in Nangarhar, Paktia and Paktika provinces near the border.

The Reagan administration, meanwhile, is stepping up its military support for the rebels by authorizing the CIA to supply them with U.S. heat-seeking antiaircraft missiles.

Statements by Gorbachev to President Reagan at their Geneva summit meeting last November led to widespread speculation that the Soviets might seriously pursue a negotiated settlement of the six-year-old Afghan war. Subsequently, U.S. sources said they saw no tangible evidence of such a move.

On Dec. 12, Deputy Secretary of State John C. Whitehead announced U.S. readiness to serve as an international guarantor of a negotiated settlement that would include a withdrawal of Soviet troops and an end to outside aid to the Afghan rebels.

During a Dec. 16-19 session of U.N.-sponsored indirect talks in Geneva, the Afghan government representative indicated he possessed details of a Soviet withdrawal plan but said he would present them only in face-to-face discussions with the Pakistanis. A report that Cordovez had been permitted to see a timetable at that time was denied by all sides.