Although fighting between Iranian and Iraqi forces appeared to be less intense yesterday, there were indications the Iranians may be planning a much larger invasion further to the north within the next week, according to both U.S. officials and Arab sources.

Intelligence reports reaching here indicated that fighting on the second day of Iran's invasion of Iraq was continuing along about a 15-mile wide front line. They also indicated that the initial Iranian thrust across the border in southeastern Iraq Tuesday night had penetrated about six miles into Iraqi territory, roughly opposite the Iraqi port city of Basra.

The six-mile estimate is less than the 12 miles estimated here yesterday. But sources say these changes reflect refinements in intelligence estimates based on data that always lag many hours behind. Officials said there was not much hard, or very precise, intelligence being received from the area.

Iraq claimed yesterday that a three-pronged counterattack had driven the invading Iranian forces from Iraqi soil. But American officials monitoring intelligence information developed independently--presumably from picture-taking satellites, high-flying reconnaissance planes or airborne warning and control systems radar planes based in Saudi Arabia--say the Iraqis have not pushed the Iranians back and that the ground war is being fought inside Iraqi territory.

Analysts here say the Iranians probably penetrated Iraqi defenses more quickly than they anticipated in their initial thrust across the border. They said initial Iraqi resistance was fierce but not prolonged and that the Iraqi defense line was both static, meaning the defenders did not move around well, and had large gaps that were exploited by the Iranians.

Since the initial attack and Iranian gains, sources here said the Iraqi lines appear to be holding but it was not clear if this was because they had regrouped or if Iranians forces had halted on their own.

The Iraqis have a larger air force than Iran but their planes are not equipped for night combat and the Iranians have done most of their attacking at night.

As analysts here began to study the intelligence reports, some said it appeared the initial Iranian invasion was a strong assault by several thousand troops but may not be the main attack.

The total number of Iranian troops in the general vicinity of the southern border between the two warring Persian Gulf states is variously estimated at between 80,000 and 120,000 by U.S. officials, depending upon how much territory is included. They face an estimated 100,000 Iraqi soldiers.

Some military analysts, however, believe the Iranians are planning a much bigger attack to the north of Basra that would be a flanking maneuver intended to trap large numbers of Iraqi troops in the region between Basra and the Iranian border.

Government specialists here speculated that the attack would come in conjunction with the Islamic holy day of al-Fitr, which marks the end of Ramadan, a month of fasting. The Iranian offensive is called "Operation Ramadan."

Usually well-informed Middle East sources in Washington also told reporters privately several days ago that, based on information available to them, a large-scale Iranian invasion would coincide with this holiday, which is next Wednesday.

The idea is to inflict as much damage to the Iraqi army as possible or to capture large numbers of troops in a maneuver that could humilate the Baghdad government of Iraqi President Saddam Hussein.

American officials said they had no reliable estimates of the numbers killed, wounded or taken prisoner thus far in this latest battle in the 22-month-old war.

Senior American officials yesterday said there was considerable concern within the Reagan administration about the implications of a victory by the Iranian forces of Ayatollah Khomeini. The officials said "it could feed the fires of Islamic fundamentalism and put at risk the other Gulf states" and could cause the collapse of Saddam Hussein's government