(CNN) -- The United Nations chief warned Saturday that all sides will be held accountable for the killing of civilians in a besieged Syrian border city, where President Bashar al-Assad's forces and Hezbollah fighters have been battling rebels for control.
Secretary-General Ban Ki-moon's warning came as he urged forces loyal to al-Assad and rebels to allow civilians to leave the town of Qusayr, the latest flashpoint in the more than two-year-old conflict that has left about 80,000 people dead.
In a statement, Ban said "the eyes of the world are upon them, and they will be held accountable for any acts of atrocity carried out against the civilian population."
As many as 1,500 people are believed to be wounded and in need of medical care in the town, and the United Nations and the International Committee of the Red Cross called on both sides to allow the wounded to be evacuated.
Navi Pillay, the U.N. High Commissioner for Human Rights, and Valerie Amos, the emergency relief coordinator for the United Nations, called the situation in Qusayr "desperate."
"We are also receiving reports that civilian neighborhoods continue to be indiscriminately attacked, and that other major violations of human rights and international humanitarian law are being committed," the two said in a joint statement.
Qusayr is a key location because it sits astride one route to the Syrian coast and another to the Lebanese border. For the rebels, holding Qusayr is important because it's another way of strangling the regime's ability to sustain itself, and it complicates Hezbollah's access to Syria.
But the fight for control of Qusayr has also raised fears the Syria conflict could destabilize Lebanon, with news that Hezbollah fighters have joined al-Assad's forces in laying siege to the city.
At least 16 rockets were fired from Syria into Lebanon early Saturday, NNA, the Lebanese state news agency, reported.
While the rockets struck an open field and did little damage, according to NNA, it was considered the largest scale assault by rebels who vowed to retaliate when Hezbollah fighters joined in the fight for Qusayr.
Word of the attack came as reports surfaced that al-Assad's government will get at least 10 fighter jets from Russia, raising concerns the move would further damage already rocky diplomatic efforts to try to bring about an end to the Syrian civil war.
A source involved in Russia's defense complex told the official Itar-Tass news agency Friday that Russia will sell al-Assad's government MiG-29 fighter jets to fulfill a "contract ... concluded long ago."
A Syrian delegation in Moscow is pressing for even more jets, with MiG executive Sergei Korotkov telling Russia's state-run RIA Novosti's news that his company and these Syrian authorities are talking about the "details and time frame of a possible contract."
Such discussions come on the heels of the European Union's decision to end its Syrian arms embargo. That move was seen as opening the door for European nations to help rebel fighters militarily, though it may give Russia more wiggle room to back its longtime ally, Assad.
It's not clear how weapons, fighter jets and ammunition flowing into Syria will affect the fighting there, much less the heretofore unsuccessful attempts to forge a negotiated settlement.
Russia also reportedly has sold six S-300 air defense systems to Syria under a 2010 contract.
Moscow, however, has said such deliveries would conform with international law and has denied supplying Syria with weapons that can be used against civilians.
U.S. Secretary of State John Kerry described any transfer of S-300s to the region as "not helpful."
"Whether it's an old contract or not, it has a profoundly negative impact on the balance of interests and the stability of the region, and it does put Israel at risk," he said.
Kerry added that halting the bloodshed peacefully should be everyone's first priority.
CNN's Tom Watkins and Greg Botelho contributed to this report.