KARACHI, June 28.--Pakistan's President Yahya Khan announced today that he had appointed a "committee of experts" to draft a new constitution that would return his country to civilian rule "in a matter of four months or so."

However, he said in a nationwide broadcast in English, "The precise timing will naturally depend on the internal and external situation at the time."

It was the process of devising a constitution to restore civilian rule that brought to the surface deep conflicts in East and West Pakistani views of regional-federal relations including what Yahya has described as secessionist tendencies in East Pakistan. In late March, of the eve of the convening of the constitutional assembly, Yahya ordered the Pakistani army into East Pakistan to prevent what he claimed was a move toward secession. This action resulted in bloody clashes and the flight of more than 5 million East Pakistanis to India.

Yahya said that the "cover of martial law" would be at the disposal of the national and provincial governments "for a time," adding that actual practice, Martial law would not be operative in its present form. He did not elaborate on what he meant by this.

Yahya ruled out, in effect, the formation of any East Pakistan-based political party to replace the now-banned Awami League saying that under guidelines he had given the constitutional committee the new constitution would not permit the existence of any political party which is confined to a specific region and which is not national in scope.

He stressed that the results of the provincial and national elections held last December and January would stand, although there would be by-elections to fill the seats of some Awami League leaders who were disqualified for engaging in "antistate activities."

The president said that the new constitution, the third in Pakistan's 24 years of independence, would be promulgated on the day the National Assembly is called into session, and that the assembly - -which had originally been supposed to write the constitution - would be able to amend it according to procedures to be spelled out in the constitution itself.

Yahya, who has ruled Pakistan by martial law since the fall of the 10-year regime of Ayub Khan in 1969, said that the new constitution would give the provinces "maximum autonomy including legislative, administrative and financial, but the federal government shall also have adequate powers including legislative, administrative and financial to discharge its responsibilities in relation to external and internal affairs...."

Sheikh Mujibur, whose Awami League had won an absolute majority in the December elections, was among those jailed following the Pakistani army's March 25 crackdown in East Pakistan.

Yahya's government has been under increasing international pressure since the army crackdown. Most recently, the major aid-giving nations, led by the World Bank decided last week to postpone indefinitely any new economic assistance to Pakistan.

But news reports indicate that trouble in the area is still continuing, though with less intensity than immediately after the army moved in. An AP dispatch from Dacca, capital of East Pakistan, today quoted "reliable sources" as saying that the army had attacked five villages within the past four days killing Hindu men and burning homes and markets in predawn raids.