JOSEPH EID / AFP / Getty Images Syrian troops take control of the village of Western Dumayna, some 7 km north of the rebel-held city of Qusayr on May 13, 2013

Fighting continued for a fifth straight day in the strategic Syrian city of Qusayr, as opposition forces fighting the regime of President Bashar Assad sought desperately to maintain their slipping grip in a battle that could dictate the direction of the war. As government tanks, artillery and warplanes pounded rebel positions throughout the city, fighters engaged in sniper attacks and small forays against government ground troops seeking to take terrain. Rebel fighters are calling it one of the worst ground battles of the war.

“In some areas the fight is within three-meters diameter, and you are able to hear them yelling and crying in the battlefield,” Abu al-Baraa, a field commander from the Jabhat al-Nusra in Qusayr, tells TIME by telephone from Tripoli, where he is recovering from wounds gained earlier in the week. Al-Qaeda-affiliated Jabhat al-Nusra is fighting under the umbrella of the Free Syrian Army, a loose confederation of volunteers, jihadists and opportunists aligned against the regime. Al-Baraa says he is in constant contact with his men at the front. “Now the battle is centralized in the eastern side of Qusayr, and our mujahedin are teaching [the Syrian government forces] hard lessons which, God willing, they will never forget.” He says his brigade has more than 2,000 fighters willing to sacrifice their lives to prevent Qusayr from being taken back by regime forces. Assad’s supporters are equally ferocious in their desire to retake the city, which has been under rebel control for several months.

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Qusayr, a city of 30,000, straddles a key transit corridor between the Syrian capital of Damascus and the coast. Victory in Qusayr allows the regime easy access to the Mediterranean port city of Tartus, where Russian tankers can supply both oil and weapons in case the Damascus airport is destroyed. Tartus is also the entryway to a coastal region dominated by Assad’s Alawite sect — an essential refuge for the President and his supporters should Damascus fall. “No doubt Qusayr is a strategic city for the Rafidah,” says al-Baraa, using a derogatory name for Alawites, meaning rejecters, or apostates. “It is the main city that will allow them to link their state together.”

For the rebels, Qusayr is an important logistics hub. Weapons and supplies can easily be smuggled over the porous Lebanese border, 10 km away, and fighters, like al-Baraa, use safe houses across the border for rest and recovery. Members of Hizballah, an Iranian-linked Shi‘ite militia based in Lebanon, have gone across the border in the opposite direction to help the regime, raising fears of a regional and sectarian conflagration. With more than 80,000 killed in a civil war that has gone on for more than two years, the fight for Qusayr is seen as a pivotal test by both sides. George Sabra, acting head of the opposition Syrian National Coalition based in Turkey, reiterated the importance of Qusayr in a statement calling for reinforcements of men and weapons on Wednesday, citing concerns about sectarian violence and “foreign invaders” from Hizballah and Iran. “Everyone who has weapons or ammunition should send them to Qusayr and Homs to strengthen its resistance. Every bullet sent to Qusayr and Homs will block the invasion that is trying to drag Syria back to the era of fear.”

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In a war where journalists have limited access, the propaganda battle of Qusayr is equally vociferous. The government news agency SANA claims to have taken half the city, whereas a local government official from the district governor’s office told the Associated Press that 80% of Qusayr was in government hands. Rebel fighters in the western part of the city, where the fighting is most fierce, told TIME by Skype that the FSA has 60% of the city. “We still control the center of Qusayr and the west,” says activist Abu Islam, speaking from the FSA’s Qusayr media center. As proof, he pointed out that the media center was able to run on generator power and he could speak safely using the Internet. On Twitter, rebels, activists and government supporters traded taunts and crowed over successes. “So far we have the bodies of 50 pigs,” al-Baraa told TIME, using an extremely pejorative term for anyone of the Muslim faith. Then, using a play on words that twisted the meaning of Hizballah, or Party of God, into party of idol worshipers, he said “Hizb el-Lat already lost 70 fighters under the strikes of our mujahedin, and if they continue the number will be more than 700.” Others in Qusayr projected weary defiance. One doctor, filmed at a makeshift field hospital where he attended wounded civilians and rebel fighters, said that half the houses in the city had been destroyed, that scores had been injured and that there was a shortage of “everything.” Still, he declared: “We will not surrender, and Qusayr will not fall.”

U.S. Secretary of State John Kerry, in a meeting with the Friends of Syria group in Amman on Wednesday, admitted that the regime had “made some gains in the last few days but this has gone up and down like a seesaw.” Kerry told reporters that Assad was “miscalculating” if he thought the advances would be decisive. But hopeful predictions that the regime is on its last legs are just that — hopeful. For nearly a year observers have spoken of the government’s imminent collapse, only to be proved wrong again and again. As long as Syria has the support of Iran and Russia, it is unlikely to fall no matter how well armed the rebels.

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Victory in Qusayr, for either side, will have long-term implications, not just for Syria but for the region. The U.S., Russia and the international community are preparing for a summit next month that hopes to find a political solution to the Syrian conflict. By reasserting its military superiority in Qusayr, and by extension the west of the country, before the summit, the regime will be able to transform its military advances into a stronger negotiating position. For Russia this means keeping Tartus, its only warm-water port, in the hands of a close ally, even if the rest of the country falls to the rebels. For Iran, it means keeping a conduit open to its proxy, Hizballah. For that reason alone, Israel will keep a close eye on what happens next in Qusayr. “There are several thousands of Hizballah militia forces on the ground in Syria who are contributing to this violence, and we condemn that,” said Kerry at the Friends of Syria meeting, referring to Qusayr. If the regime can consolidate power from Damascus to the coast in a swath of territory that flanks northern Lebanon and Hizballah’s Lebanese heartland of the Bekaa Valley, the opportunity for weapons transfer from Iran to the sworn enemy of Israel via the Syrian capital will be even stronger. The Israelis have already targeted Syrian military positions three times under the suspicion that they were being used as transit points for weapons destined for Hizballah in Lebanon.

But the greatest risk of a regime success in Qusayr would most likely fall on its vulnerable neighbor, Lebanon. The conflict has already spilled into the northern Lebanese city of Tripoli, where Alawites and Sunni Muslims have fought pitched street battles, killing 14 since Sunday. Rebel commanders in Syria, and Sunni religious leaders in Lebanon, have hinted at sectarian revenge attacks against Shi‘ites and Alawites on both sides of the border should Qusayr fall, laying the groundwork for a regional sectarian conflict.

— With reporting by Rami Aysha / Beirut