Iranian forces have succeeded in reinforcing the embattled port of Khorramshahr and the burning oil refinery complex at nearby Abadan, and Iranian artillery has begun to pound surrounding Iraqi positions along the disputed Shatt-al-Arab waterway.

Tehran also announced today that it has dropped paratroopers into Khorramshahr as part of a counteroffensive to drive out the invading Iraqi forces.

Signs of the new Iranian activity were clearly visible from this Iraqi port at the southern point of the Shatt. New Iranian artillery batteries were laying down heavy fire against Iraqi positions behind palm groves across the waterway and up its left bank toward the main Iraqi troop and tank concentrations.

Whether the renewed Iranian vigor would be enough to stall the ponderously slow Iraqi advance on the port city was still a matter for great conjecture.

Both Iraq and Iran spoke of attacks and counterattacks around Khorramshahr today, and though independent on-the-scene assessment of those claims was hard to obtain, it was clear that the battle for the port city has reached a new intensity.

Viewed from the sheltered palm groves across the river today, Khorramshahr was evidently receiving its greatest shelling of the 11-day-old war, despite the new Iranian artillery ripostes.

Where only one major fire was burning in the port city at noon, nine other major conflagrations had been touched off by Iraqi shellings by midafternoon, sending columns of new smoke almost as high as the flames and smoke from the oil installations in neighboring Abadan.

In Baghdad, the government-controlled media said Iraq had mounted a fresh assault on Khorramshahr and declared, "Savage battles are raging from house to house and street to street to complete the liberation of the city."

In Tehran, President Abol Hassan Bani-Sadr called upon residents of Khorramshahr to "do their best" and said the gates of the city should be turned into a "slaughter place for all Iraqi invaders."

In a radio broadcast, the Iranian president also ordered a "general mobilization" of the entire population in Khuzestan Province, calling upon it to "rise up and take part in a heroic battle" to "protect your honor and religion."

At the same time, he appealed to the armed forced to redouble their efforts to beat back the Iraqi troops, announcing a rout of enemy forces at the border town of Susangerd 100 miles north of Khorramshahr and the capture of 150 Iraqi tanks and armored personnel carriers. There was no confirmation of this claim, however.

Wednesday, Iran claimed it had recaptured another border town in the central sector, Mehran, although the Iraqi government subsequently denied it. c

The first hard evidence of Iran's counteroffensive around the besieged ports of Khorramshahr and Abadan came after Tehran's total rejection of a conditional Iraqi offer of a four-day cease-fire and as the forces of the two warring nations continued their battle for control of four key cities in the Iranina oil heartland of Khuzestan Province.

Iran also rejected yesterday yet another attempt to mediate in the dispute, this time by Cuban Foreign Minister Isidoro Malmierca who was told in Tehran by Iranian Prime Minister Mohammed Ali Rajai that Cuba "has misunderstood our revolutionary people."

meanwhile, Iraq warplanes returned to their raids on vital Iranian industrial sites, apparently scoring a rare direct hit on the sprawling $3 billion Japanese-built petrochemical complex in Bandar Khomeini 100 miles east of Khorramshahr.

Officials of Mitsui Company told reporters in Tokyo that the attack by several Iraqi fighters hit the central section of the nearly completed project and heavily damaged three of the 13 plants which make up the complex.

They said the attack, the second on the complex in a week, could seriously delay completion of the project, which had been scheduled for the spring of 1982. The 745 Japanese workers at the site had already been evacuated to a camp six miles away, the officials said.

Iraq also reported fighting around Ahwaz, the capital of Khuzestan Province, which Iraqi forces also have under siege and claim to have surrounded on all sides. An Iraqi war communique reported the capture of two Iranian barracks near Ahwaz and the destruction of 14 Iranian tanks and several missile launching ramps.

Other communiques reported Iraq had suffered 16 dead and 24 wounded in fighting along the northern and central parts of the 300-miles front but did not indicate where these casualties had taken place. The Iranians had suffered "a large number of dead and wounded," one communique said.

They also said Iraq had lost seven tanks, three armored personnel carriers and 12 other vehicles.

In the midst of these Iraqi reports of heavy fighting in their attempt to take four main Khuzestan cities -- Khorramshahr, Abadan, Dezful and Ahwaz -- and other accounts of Iraqi troop reinforcements and new artillery rolling toward the war front, the Iraqi High Command issued a communique late tonight suggesting a change in strategy.

It said that "since our armed forces have achieved their main targets, their operations will be dedicated now to perserving these targets." There was no other clarification of the statement, which seemed to imply that Iraq was ending its offensive before having taken any of the cities to which it has laid siege over the past 10 days.

In other action in the sputtering air war, Iraq reported new raids on Iranian military targets, including four attacks around Dezful while Iranian warplanes hit sites in five Iraqi provinces. It said two more Iranian jets were shot down over Amara and Basra, bringing the total Iranian losses, according to Iraqi calculations, to over 200 jets.

Iranian press reports, on the other hand, claim Iraq has now lost 101 Migs as well as 510 tanks in the war so far.

Echoes of the war reached as far as Beirut, where the Iranian and Iraqi embassies were both hit by rocket fire, causing considerable damage but no injuries. An Iranian Embassy spokesman blamed the attack on members of Iraq's ruling Baathist Part living in Lebanon or the pro-Baghdad Palestinian Arab Liberation Front.