Iraqi warplanes bombed residential neighborhoods of Tehran yesterday after an explosion destroyed the headquarters of a 12-story Iraqi government bank in downtown Baghdad, apparently the target of saboteurs working for Iran.

Both nations hit several other civilian targets in an escalation of their 10-day-old "war of the cities."

The adversaries continued to fight bloody ground battles on the strategic marshlands in the southern sector of the border warfront, and an Iraqi jet hit another oil tanker in the Persian Gulf just loaded near Iran's oil export terminal at Kharg Island. Iran's official IRNA news agency said an Exocet missile scored a direct hit on the Panamanian-registered tanker and set it ablaze. The fate of the crew of about 25 was not known.

An Iraqi military communique said the retaliatory raids on Tehran covered mainly the "city center where the chief charlatan Khomeini lives," referring to Iranian revolutionary leader Ayatollah Ruhollah Khomeini. In Tehran, military spokesmen said three rockets fired on the Iranian capital killed at least three persons.

That attack followed by about two hours an explosion in Baghdad that wrecked the government-owned Rafidan Bank. Iran claimed to have caused the blast by firing a single surface-to-surface missile from inside Iranian lines more than 100 miles from Baghdad. Informed sources in Washington tended, however, to believe the version given by Iraq, It said pro-Iranian agents operating in Baghdad had planted the explosive charges inside the bank headquarters. No casualty figures were available.

Iran, meanwhile, renewed its threat to start using chemical weapons in the fighting in the marshland invaded by Iranian forces early on Monday. Iran apparently sought to isolate the key river port of Basra, which with a population of about 1 million is Iraq's second-largest city. Iran charges Iraqi use of chemical weapons three times in the marshlands, news services reported.

This latest upsurge in the 4 1/2-year-old war accompanied unsuccessful efforts by U.N. officials to arrange for a halt to the bombing of cities and adherence to an accord, mediated by the United Nations last June, not to attack civilian targets.

In Bahrain yesterday, U.N. General Assembly President Paul Lusaka of Zambia said he could see no early end to the fighting. "I can see only escalation and I can't see hope," Lusaka said at a news conference. "We thought we were going to have a breakthrough a few days ago but things went the other way."

Lusaka said the United Nations could arrange a peace-keeping force but only if Iran and Iraq agreed to a cease-fire.

Although Iran and Iraq blamed each other for the recent escalation in the fighting, it comes as Iraq has begun to take a more aggressive military stance.

According to Arab and western sources here, Khomeini appears to remain adamant in his demand that Iraqi President Saddam Hussein be toppled as Iran's price for ending the war. But these sources also noted with interest signs of a growing frustration among Iranian Army officers and Revolutionary Guards with the manner in which the draining war is being waged.

In addition, one of the sources said, there are faint hints that a minority bloc around Khomeini is beginning to argue for the first time for a negotiated settlement.

There is also considerable interest here in the activities in Tehran of Khomeini's initial prime minister, Mehdi Bazargan, who has stayed in the capital. The Manchester Guardian reported yesterday that Bazargan and many of his former Cabinet colleagues have signed a two-page letter circulating underground in Tehran warning that the "many major crises" in Iran have combined to "create a national anxiety" and led to a "lack of basic freedoms."

Reports reaching here suggest that in recent months Iran has seen inflation, high unemployment and shortages of basic goods. Sources here say Iran's foreign exchange reserves have dwindled to half or less of their level of $8 billion to $10 billion last year, because of slackened demand for oil and the discounts on prices that Iran gave after Iraq began the tanker war last summer.

There have also been reports of worker strikes in an oil refinery in the city of Isfahan and at Iranian steel plants that were said to have been quelled by force.

For more than a year now, Iran has been threatening a "final offensive" against Iraq and has maintained more than a quarter of a million troops massed at the southern front. Sources here question whether Iran is capable of a sweeping offensive because of Iraq's superiority in weaponry. They expressed confidence that Iraq would eventually push Iranian forces out of the marshland in the south.

Iranian artillery pounded Basra yesterday and an Iranian military statement said its warplanes had attacked five northern Iraqi cities. Iran confirmed Iraq's claims that Iraqi jets had strafed the northern Iranian city of Tabriz.