UNITED NATIONS—Russia said it has "clear" evidence that Syrian rebels fired a rocket laden with sarin gas at an Aleppo suburb in March, a view that clashed with U.S. conclusions on the same issue last month and was initially rejected Tuesday by Washington.

The Russian claims were contained in a confidential 80-page report delivered on Tuesday to U.N. Secretary-General Ban Ki-moon, said Russia's ambassador, Vitaly Churkin.

Mr. Churkin said Russian experts had direct access to the impact site at Khan al-Assal and that evidence taken to a Russian laboratory showed the lethal sarin gas had not been industrially produced but was made in a "makeshift" facility. He also said the rocket used in the attack, a Basha'ir-3, wasn't standard for chemical weapons and that production of the rockets was begun in February by the Basha'ir al-Nasser Brigade, affiliated with the opposition Free Syrian Army.

U.S. officials concluded in June that forces loyal to Bashar al-Assad's regime had used chemical weapons at Khan al-Assal that day, as well as on April 13 at Sheikh Maksud, a neighborhood in Aleppo, on May 14 at Qasr Abu Samra and on May 23 in Adra.

The U.S. finding, which was dismissed by Moscow, was announced at the same time the Obama administration decided to begin providing arms to Syrian rebels. The White House said Tuesday it had no reason to alter its view.