UN Secretary General Ban Ki-moon said he was “extremely concerned” over Hezbollah’s involvement in the Syrian conflict late Sunday, calling on the Lebanese terror group to respect Syrian sovereignty.

The statement came as members of the Shi’ite militia continued to fight alongside Syrian troops against rebels in the border town of Qusair, and hours after a Hezbollah stronghold in Beirut was shelled, apparently in response to the group’s support for Syrian President Bashar Assad.

The fighting in Qusair, which began with a regime offensive last week, has marked the most overt Hezbollah involvement in Syrian fighting to date, and has been accompanied by statements from group head Hassan Nasrallah that he will protect Assad’s embattled rule.

“The secretary-general is deeply concerned by the acknowledged increased participation in the fighting in Syria by Hezbollah, as well as by the risk of spillover in Lebanon, which has witnessed growing tension over the past week,” Ban’s office said in a statement.

“The Secretary-General continues to be extremely concerned by the intense military confrontation in Syria, in particular in recent days in and around the town of Qusair, and the resulting human toll and suffering,” the statement read.

Earlier Sunday, mortars slammed into Hezbollah areas in Beirut, injuring four Syrians.

There was no claim of responsibility, but the attack was widely portrayed as retaliation for Nasrallah’s defiant speech and Hezbollah’s participation in a regime offensive in the past week on Qusair. The regime has pushed back the rebels in Qusair, but has so far failed to dislodge them.

In an amateur video posted online a few days ago, a rebel commander threatened to hit Hezbollah targets in south Beirut in retaliation for the militia’s part in the fight for Qusair.

On Saturday, Nasrallah vowed in a televised speech that Hezbollah would keep fighting alongside Assad’s forces until victory came, regardless of the costs.

At the World Economic Forum meeting in Jordan Sunday, Salman Shaikh, director of the Doha Center of the Brookings Institution think tank, said Hezbollah had 5,000 troops fighting inside Syria and another 5,000 were readying to enter the neighboring country.

Last week, French Foreign Minister Laurent Fabius said he would reverse Paris’s position and support labeling Hezbollah a terror group, in light of the organization’s involvement in backing Assad.

The Associated Press and David Horovitz contributed to this report.