BEIRUT (AP) — Syrian rebels captured a government checkpoint on the main road to the country’s second-largest airport Monday as opposition fighters pressed their campaign to capture the strategic facility in the northern city of Aleppo, activists said.

The rebels launched a major attack on the airport and the adjacent Nairab military airfield last week, and have since overrun the main army base protecting the area. Control of the airport would provide a significant boost for the opposition, and mark a strategic shift in the country’s 23-month civil war.

The fighting has been raging for weeks, but it was only in early February after the rebels captured the strategic Aleppo neighborhood of Sheik Said near the airport, which has been shut down because of the violence, that the offensive gained momentum.

The Britain-based Syrian Observatory for Human Rights and the Aleppo Media Center reported that rebels captured the main fuel station on the Nairab base. The Observatory said there were several regime casualties and five rebels, including foreign fighters, killed before rebels withdrew from the station a few hours later.

The fight for the international airport in Aleppo, Syria’s largest city and commercial hub, highlights the importance that both sides in the conflict place on controlling strategic assets that could provide an edge in the larger fight for the country.

Last week, rebels seized a hydroelectric dam and a major oil field, cutting off President Bashar Assad’s regime from key resources necessary for its long-term survival. Rebels also seized an air defense base and fought near two other army installations in Syria’s north.

Aleppo’s airport is crucial in the broader fight for the city itself. Rebels launched an offensive on Aleppo in July, and quickly seized several neighborhoods. Since then, the fighting has settled into a bloody stalemate that has destroyed entire districts, killed thousands and forced thousands more to flee their homes.

Determined to hang onto the city, Assad’s regime has thrown troops and resources into the fight. But his army has faced difficulties in sending reinforcements to Aleppo since October, when rebels captured the strategic town of Maaret al-Numan, about 80 kilometers (50 miles) southwest of Aleppo. Maaret al-Numan is on the highway that links Damascus with Aleppo.

The regime has not been able to fly supplies since the Aleppo airport closed weeks ago due to the fighting.

Rami Abdul-Rahman, who heads the Observatory, said over the past two weeks large army reinforcements were able to reach the town of Safira, near the city of Aleppo, after securing a desert road that links the central province of Hama with northern Syria.

He said the reinforcements, including dozens of vehicles and thousands of troops, have not been able to reach Aleppo because of heavy fighting in the town of Tal Aran between the city’s international airport and Safira.

Safira is home to military production facilities that rebels have been trying to capture in recent past weeks with no luck.

“The regime is doing all it can to open a road to Aleppo,” Abdul-Rahman said.

Abdul-Rahman, whose group relies on a network of activists around the country, said as the army convoys pushed north, more than 200 members of al-Qaeda-linked Jabhat al-Nusra were killed over the past two weeks in clashes with the reinforcements. The Obama administration designated Jabhat al-Nusra a terrorist organization in December.

Also Monday, European Union foreign ministers said they are keeping the current sanctions against Syria in place for three months, blocking a push by some to ease restrictions so some countries could arm the rebels.

An EU official said before the meeting in Brussels that Britain was pushing to ease the embargo, but several foreign ministers from other countries said they opposed the move.

Since the opposition began taking up arms against Assad’s forces in late 2011, it has pleaded for military aid, calling it the only way to turn the tide against Assad’s forces.

The US and other countries have resisted such a move, saying there is no way to control how the arms are used, especially with Islamic radicals rising in the rebel ranks.

Bolstering those concerns, a UN-appointed panel said Monday that the civil war is becoming more sectarian and that both sides are becoming increasingly radicalized.

The panel’s 131-page report, released Monday, said regime forces and affiliated militias committed crimes against humanity such as murder, torture and rape. It said anti-government armed groups have committed war crimes, including murder, torture and hostage-taking, but said these did not reach the “intensity and scale” of the government’s violations.

It said the only way to end Syria’s crisis is a political solution between the two sides.

Also Monday, the Lebanese militant group Hezbollah announced that one of its fighters had been killed while doing “his jihadist duty.” It gave no further details on his death.

The announcement followed reports from Syrian activists and a Lebanese official near the Syrian border that at least two Hezbollah fighters had been killed in sectarian clashes near the Syrian town of Qusair on Saturday.

Hezbollah declined to comment on the clashes, and it could not be confirmed that the fighter buried Monday was killed in Syria.

The Observatory and the Local Coordination Committees activist group also said government forces were shelling southern neighborhoods of the capital, Damascus, mostly around the rebel-held Palestinian refugee camp of Yarmouk. There has been intense fighting in recent weeks in southern and northeastern districts of the capital.

A Syrian official said three mortars had fallen on the Qassaa neighborhood, a Christian area in central Damascus. Speaking on condition of anonymity because he was not authorized to brief the media, the official said two mortars hit the roof of a hospital and another fell in a school playground, causing material damages.

The UN says nearly 70,000 people have been killed in Syria’s conflict since the revolt against Assad began in March 2011.

Copyright 2013 The Associated Press.