A mortar shell slammed into a tent packed with supporters of President Bashar Assad, killing 39 people and wounding 205 others Friday in the first attack on a campaign event, and coming less than a day after the United Nations condemned the Syrian government for not allowing crucial humanitarian aid to be delivered from outside its borders.

U.N. secretary-general Ban Ki-moon said in a confidential report to the Security Council that the Syrian government is “failing in its responsibility to look after its own people” and urged the council to authorize the delivery of food and medicine to Syria without the government’s consent.

The report, presented to the council overnight Thursday, represents the strongest push to date by Ban in exhorting the council to permit U.N. agencies to deliver aid into areas held by insurgents in the three-year-old Syrian conflict. He later condemned Friday’s mortar attack and reiterated his opposition to the indiscriminate use of any weapons by any party against civilians.

Syria has consistently rebuffed U.N. appeals to permit humanitarian aid deliveries. As a result, 85 percent of U.N. food aid, and more than 70 percent of medicines, have remained in areas of Syria that are controlled by the administration of President Bashar Assad.

Syria has argued that it is within its rights as a sovereign nation to admit or deny entry to anyone, and that position has been supported by Russia, Syria’s principal ally and a permanent member of the council with veto power.

Ban disputed that contention in his report.

“On the contrary,” he said, “it is an affirmation of the sovereign responsibility of the government to ensure that its citizens do not suffer in such a tragic and unnecessary way.”

Several council members, frustrated by the blockade on humanitarian relief, are discussing a draft resolution that would open up four specific border crossings, rather than give the United Nations blanket permission to carry out cross-border aid deliveries. Those four crossings — two along the frontier with Turkey, one on the Jordan border and one from Iraq — would help the United Nations reach more than 1 million people in opposition-held areas.

The humanitarian relief issue was raised again in the council one day after Russia and China vetoed a resolution, supported by all other members, that would have referred Syria to the International Criminal Court in The Hague, Netherlands, for the investigation and prosecution of war crimes.

Meanwhile, Friday’s shelling of the campaign event underscored deep fears in government strongholds that rebels will escalate attacks in an attempt to disrupt the presidential balloting next month.

Assad is widely expected to win a third, seven-year mandate in the vote scheduled for June 3, but the West and opposition activists have criticized it as a farce since it is taking place despite a raging civil war.

The 49-year-old president himself has not made a public appearance in more than a month and was not at the gathering struck by the mortar shell late Thursday in the southern city of Daraa. But campaigning has begun in earnest, with supporters waving his pictures and Syrian flags during daily demonstrations in the capital, Damascus, the coastal city of Latakia and other government-held areas.

State TV showed pictures of Assad supporters dancing in a campaign tent in Daraa. It then showed people lying dead and wounded on the ground, including several children.