BEIRUT >> The warring parties in Syria have launched newly assertive attacks on several fronts in recent days, seeking to gain ground and a psychological advantage ahead of an intensified U.S. campaign against extremist Islamic State militants that could include the first U.S. airstrikes inside Syria.

On Tuesday, fighters with the Islamic State said they had shot down a Syrian military aircraft over their stronghold in the northern city of Raqqa, in what anti-government activists said was a first for the Islamic State.

Syrian-led insurgent groups have brought down numerous Syrian military planes and helicopters in the past, but such attacks have grown rare recently as rivals of the Islamic State struggle to maintain arms supplies. The downed plane crashed into a house, killing eight occupants, said an activist in Raqqa who refused to give his name for fear of reprisal.

Amid a three-year civil war that pits the government of President Bashar Assad against many insurgent and often rival factions, new attacks by the government in eastern Syria and by opponents of the Islamic State in Damascus demonstrated the volatility of the situation on the ground. It also showed the difficulty for the United States in striking the militants without allowing the Syrian government or al-Qaida-linked insurgents to take advantage of any weakening of the Islamic State.

While the combatants scrambled on the battlefield, the chairman of a U.N. investigatory panel on human rights said in Geneva that he “had run out of words to depict the gravity of the crimes committed inside Syria.”

In a report to the Human Rights Council, the chairman, Paulo Sergio Pinheiro, said that even as the Islamic State carries out atrocities, the Syrian government “remains responsible for the majority of the civilian casualties, killing and maiming scores of civilians daily.”

He described killing “from a distance” by shelling and aerial bombardment and “up close at checkpoints and in its interrogation rooms,” presenting a report that also included new details of killings that were the work of the Islamic State.

Pinheiro reiterated that the panel had repeatedly urged the U.N. Security Council and influential states to refer Syria to the International Criminal Court and to push for a political settlement.

Their inaction “nourished the violence” consuming Syria, he told the council. The Islamic State has been “its most recent beneficiary,” he said.

Both the Syrian government and its opponents say they are fighting the militant group, which has sought to impose a brutal interpretation of Islamic rule on areas it controls and in June swept into much of Iraq and declared an Islamic caliphate.

The Syrian government has offered to join the U.S. coalition but has said that any strikes not coordinated with Syrian officials would violate its sovereignty. However, President Barack Obama has ruled out working with Assad’s government.

The speaker of Syria’s Parliament, Mohammad al-Lahham, addressed letters to congressional leaders Tuesday urging them to rethink U.S. policy and seeking to persuade lawmakers that Syria is on the side of the United States against extremism.

He argued that the arming of any insurgents, even those the United States calls moderate, violated Security Council Resolution 2170 calling for member states to stop the flow of foreign fighters into Syria. He said Washington should share intelligence with Syria and put pressure on Turkey to better track the flow of militants into Syria and on the Saudis to stop promoting extremist Islamic ideology.

In Damascus on Tuesday, security forces carried out intensive raids in the Midan neighborhood, after insurgents not affiliated with the Islamic State infiltrated the district at dawn Monday and clashed with government troops. It was the first ground attack in months across the broad bypass highway that divides the restive suburbs from the government-controlled city center.

State media said that insurgents entered through the sewer system and that all were killed. Rami al-Sayed, an insurgent spokesman in the Yarmouk refugee camp nearby, said that fighters from Islamist groups, including the Qaida-linked Nusra Front and Ahrar al-Sham, entered from the south, attacking a checkpoint and clashing with government soldiers. Tallies of the insurgent death toll ranged from three to 18.

The Nusra Front said in a statement that it killed numerous government fighters, including four security officers who were killed after being captured.

The government appeared to take the threat seriously after more than a year in which it has solidly held central Damascus, even as insurgents lobbed mortars into the city. Security forces were carrying out mass arrests and “storming and searching houses and shops,” said Firas al-Nabisi, a resident of the nearby Zahra district.

Damascus has been accused of allowing the Islamic State to operate virtually with impunity in eastern Syria, to prove Assad’s point that he is essential to combat terrorism. Lately, however, the government has been striking back.

On Monday, the Syrian army blew up a bridge in the eastern provincial capital of Deir al-Zour, cutting off an area held by the group, according to state media and insurgent spokesmen.

Then, on Tuesday, a government airstrike in northern Syria targeted Jamal Maarouf, the commander of the Syrian Revolutionaries Front, one of the insurgent groups the United States has deemed relatively moderate.

Maarouf escaped the attack on the group’s headquarters in Deir Sonbol, in Idlib province, but it killed his wife and daughter and his deputy, Faisal Maarouf, two anti-government activists said.

–Anne Barnard and Hwaida Saad / New York Times