The U.N. secretary general said Thursday that he will launch an investigation into whether chemical weapons were used in Syria, seeking to address accusations that, if proven, could alter the trajectory of the two-year-old civil war in the country.

Speaking to reporters at the U.N. headquarters in New York, Secretary General Ban Ki-moon said his top advisers are still trying to determine the scope of the mission and the steps required to guarantee the safety of U.N. personnel during the probe.

Their immediate task will be to investigate an attack Tuesday near the city of Aleppo in which both the Syrian government and the opposition claimed the other side had used chemical weapons. But Ban hinted that the team’s mandate could be broader, saying that he hoped the mission “would contribute to ensuring the safety and security of chemical weapons stockpiles in Syria.”

The Obama administration, which has said that any use of chemical weapons by the Syrian government would be a “red line,” signaled support for a wide-
ranging investigation. Susan E. Rice, the U.S. ambassador to the United Nations, said officials back an investigation that “pursues any and all credible allegations of the possible use of chemical weapons in Syria.”

The U.N. announcement came amid signs that the Syrian war, which has killed more than 70,000 people, is threatening to spiral further into a regionwide crisis. This week, a top U.N. official said that 3.6 million people have been internally displaced in the conflict and that 1.1 million have registered as refugees in Syria’s four neighboring countries.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

The war is also threatening the 39-year-old cease-fire between Syria and Israel, as fighting undermines the capacity of international peacekeepers to monitor the separation line between the two countries. Ban has warned the Security Council that Syrian armed forces have crossed into the U.N.-monitored demilitarized zone in pursuit of rebel groups using the area as a haven.

The breaches represent a “grave violation” of the cease-fire, Ban said in a letter, a copy of which was obtained by The Washington Post.

The violence in Syria, meanwhile, has continued unabated. On Thursday, a bombing at a mosque in the capital, Damascus, killed 42 people, including a senior Sunni cleric, according to Syrian media reports and opposition groups. Mohammed Said Ramadan al-Bouti was the imam of the the largest and most important Sunni mosque in the country and was known for his support of President Bashar al-Assad.

The attack that is the immediate focus of the U.N. investigation occurred in Aleppo province and is said to have killed 26 people, including 16 Syrian soldiers. There has been no independent confirmation that chemical weapons were used, nor has there been confirmation that such munitions were used in some other recent cases, as alleged by the opposition.

An administration official, speaking on the condition of anonymity because of the sensitivity of the issue, said ongoing analysis of the incident indicates that it probably did not involve the use of chemical weapons. One possibility under discussion is whether a missile struck a nearby chemical-weapons site and released a small amount of a chemical agent.

President Obama said Wednesday that he was “deeply skeptical” of the Syrian government’s assertions that the opposition had used chemical weapons. Syria is thought to possess the world’s third-largest stockpile of chemical arms after the United States and Russia — including arsenals of the deadly nerve agents sarin and VX — and there has been no evidence that the opposition has gained access to the stockpiles.

Still, Syria urged the United Nations to undertake an “impartial, independent” investigation into its claim that rebels had used the weapons Tuesday.

Western officials are calling for Ban to expand the investigation to include allegations of chemical-­weapons use dating to December.

“We wish to bring to your attention recent allegations from various sources that chemical weapons have been used in Syria, including in two locations of Khan Asal in Aleppo, and Ataybah in the vicinity of Damascus, resulting in civilian deaths and serious injuries,” France’s U.N. ambassador, Gerard Araud, and Britain’s deputy U.N. envoy, Philip Parham, wrote in a letter to Ban. “We also wish to bring to your attention allegations of use of chemical weapons in Homs on 23 December 2012.”

Russia’s U.N. envoy, Vitaly Churkin, denounced the European initiative as a stalling tactic and insisted that Ban limit the immediate investigation to the single case in Aleppo.

“There is just one allegation of the use of chemical weapons,” Churkin said. “This is really a way to delay the need for immediate, urgent investigation of allegations pertaining to March 19 by raising all sorts of issues.”

At the news conference, Ban said he has the authority to act without the approval of the Security Council. He said the mission he will authorize “is to look into the specific incident brought to my attention by the Syrian government. I am, of course, aware that there are other allegations of similar cases involving the reported use of chemical weapons.”

Ban’s warning about the war’s impact in the Golan Heights underlined that the conflict has become perhaps the most serious threat in decades to the strained, but stable, stalemate between Syria and Israel. In recent months, the secretary general said, members of the U.N. Disengagement Observer Force, which has more than 1,000 peacekeepers under its command, have been shot at, abducted, harassed during patrols and robbed — both by Syrian armed forces and the opposition.

The turmoil has forced some U.N. monitors to halt patrols and shutter observation posts. Some nations, including Croatia and Japan, have decided to pull their troops out of the Golan Heights, leaving those remaining without the means to fulfill their mission.

“The situation is deteriorating,” Ban warned in his letter, which is expected to be discussed by the Security Council next week. “The continued military activities in the area of separation have the potential to escalate tensions between Israel and the Syrian Arab Republic, and jeopardize the cease-fire between the two countries and the stability of the region.”

Babak Dehghanpisheh and Ahmed Ramadan in Beirut contributed to this report.