TEL AVIV — Israel declared Tuesday that it had found evidence that the Syrian government repeatedly used chemical weapons last month, arguing that President Bashar al-Assad was testing how the United States and others would react and that it was time for Washington to overcome its deep reluctance to intervene in the Syrian civil war.

In making the declaration — which went somewhat beyond recent suspicions expressed by Britain and France — Israeli officials argued that President Assad had repeatedly crossed what President Obama said last summer would be a “red line.” But Obama administration officials pushed back, saying they would not leap into the conflict on what they viewed as inconclusive evidence, even while working with allies on plans to secure the weapons if it appeared they were about to be used or handed to Hezbollah.

The declaration from Israel’s senior military intelligence analyst was immediately questioned in Washington. Officials said an investigation was necessary, but added that American intelligence agencies had yet to uncover convincing evidence that an attack on March 19, and smaller subsequent attacks, used sarin gas, a deadly agent that Syria is believed to hold in huge stockpiles.

“We are looking for conclusive evidence, if it exists, if there was use of chemical weapons,” Jay Carney, the White House press secretary, said when pressed on the Israeli assessment.