JERUSALEM — With Egypt’s new Islamist president headed to Iran next week and its military deploying tanks in the Sinai Peninsula — possibly outside the parameters of his nation’s 33-year-old treaty with Israel — officials here are increasingly worried about what has long been their most critical regional relationship.

Israel’s Defense Ministry and military have each sent several messages of concern to Cairo in recent days about Sinai, and received no response, a senior government official said Wednesday. That breakdown in communication, two weeks after a deadly terrorist attack along the border between the nations, comes alongside President Mohamed Morsi’s announcement that he will defy the West and break with Egyptian precedent to attend a summit meeting of nonaligned nations in Tehran, complicating Israeli and American efforts to define Iran as a pariah state because of its nuclear program.

When Egypt’s longtime leader, Hosni Mubarak, was toppled last year, Israel worried about the loss of a dependable strongman who had helped preserve a reliable if chilly peace. Concern grew with this spring’s election of a president from the Muslim Brotherhood and deteriorating security in the Sinai Peninsula, which bridges the two nations. The concerns have grown as Egyptians from across the political spectrum have in recent weeks demanded a review of the treaty, and in particular, its restrictions on Egypt’s military presence in Sinai.

While Mr. Morsi announced Wednesday that he would visit Washington next month, the gesture to the Obama administration was somewhat clouded by his decision to go first to Iran, even as Israel has implored others to boycott the meeting in solidarity with its concerns over Iran’s disputed nuclear program. And while the tanks in the desert are little threat to Israel, the lack of coordination around their deployment is seen as potentially undermining a peace treaty that has been a cornerstone of Israel’s security for decades.