Iraqi warplanes and missiles attacked more than a dozen Iranian cities and towns Wednesday night and yesterday as the Baghdad government continued its aggressive new campaign to force Ayatollah Ruhollah Khomeini's government to negotiate an end to the draining 4 1/2-year-old Persian Gulf war.

Tehran radio reported that the Iranian capital and the cities of Tabriz in the north and Khorramabad were among those bombed. There were at least 50 casualties in the air raids and "destruction of a large number of residential buildings, according to the Iranian military command.

Iraq's aggressive use of its overwhelming air superiority to break the most recent offensive and bomb the lightly defended towns and cities of Iran -- in defiance of a June 1984 United Nations-brokered accord in which both sides agreed not to hit civilian targets -- represents a new strategy aimed at draining the morale of Iranians and, at the same time, attracting renewed world attention to the war, according to Arab and western diplomats here.

Yesterday, news services reported, Iraq warned residents of Ahvaz, in southwestern Iran, that it plans to barrage the city with bombs and missiles beginning at noon Sunday and continuing until "the economic installations on the periphery of the city have been destroyed." According to Agence France-Presse, Ahvaz, about 50 miles from the Iraqi border, is the main rear base for Iran's operations on the southern front and the site of a number of military installations.

Iran responded to the latest attack on its cities with renewed shelling of the beleaguered southern Iraqi city of Basra and pounded other towns on the border. There were no details of casualties or damages inflicted.

In the Iraqi marshes on the southern battlefront where an Iranian attack touched off a week of some of the heaviest fighting in the war, new fighting was reported yesterday after a two-day lull, according to military communiques issued in Baghdad. On Monday, Iraq claimed an "epic" victory in turning back Iranian forces after they had reached the Baghdad-Basra highway, threatening to cut that vital link.

Describing yesterday's fighting as a counteroffensive in an area identified only as "east of Basra," the Baghdad communiques claimed that Iraqi ground forces had gained new territory and inflicted heavy casualties in two "daring, deep penetration" attacks. One European diplomat here said his information was that the operation was aimed at Iranian troops infiltrating back into the marshes. Tehran radio has broadcast statements threatening to launch a new ground offensive against Iraq "over the coming few days."

Sources here said that in recent weeks Iraq has doubled or tripled the number of its air missions over Iran.

Iraq said yesterday that it shot down two of Iran's U.S.-made F5 jet fighters in the first air battles since Baghdad declared Iranian airspace a war zone, United Press International reported.

Diplomats said that Iraq, fed up with "piecemeal" agreements with Iran such as the June ban on civilian targets and agreement not to use chemical weapons, is now demanding that Iran go to the bargaining table for a "comprehensive settlement" of the war as its price for ending the air raids.

Iraqi President Saddam Hussein repeated the ultimatum after meeting yesterday with a special Indian envoy seeking to stop the fighting.

Arab diplomats and U.S. officials said Iranian envoys at the United Nations have begun to show some signs of flexibility but still have not gone as far as Iraq is demanding.

However, statements by the 83-year-old Khomeini broadcast on Iranian radio yesterday and on Wednesday revealed no indication of any new conciliatory attitude. He remained adamant in insisting that Saddam Hussein be ousted if the war is to end.

But, reflecting the bitter tone of other commentaries, the Iranian leader complained that "East and West are against Iran because they fear the small wave of Islamic fundamentalism in Iran will expand." Among the other statements broadcast was one complaining that "the whole world seems to be fighting Iran" and alleging that Iraq had received help from U.S. surveillance planes stationed in Saudi Arabia.

In New York, the Iranian envoy to the United Nations, Rajaie Khorassani, said that unless Iraq ceases using chemical weapons, Iran will retaliate in kind.