Scores of Republican members of Congress have written to Barack Obama accusing him of aiding Tehran's alleged pursuit of nuclear weapons by emphasising diplomacy rather than military threats.

The letter says the president's "reticence" to attack Iran may force Israel to do so, and that if it does, Obama should offer a "crystal-clear" commitment of assistance to the Israelis.

The letter, signed by 86 members of Congress, accuses Obama of "wavering and equivocal support" of Israel in the face of a "growing, serious threat" to its existence. It questions the president's commitment to use force as a last resort against Iran – described by Obama as putting "all options on the table" – and says that his positions are undercutting Israel's ability to defend itself.

"Although your administration has stated that 'all options are on the table', you have offered little evidence to back up this statement. Your administration has consistently downplayed and even potentially frustrated Israel's understandable consideration of the use of force to protect itself," it reads.

The letter says Obama's diplomatic efforts to pressure Iran to open up its nuclear programme have allowed Tehran to buy time to work towards a weapon.

"Your equivocation has provided Iran valuable time to continue developing its nuclear programme. Israeli leaders now fear that your insistence on talking with the Iranian regime has had reverse consequences and has actually afforded Iran sufficient time to advance its nuclear goals to the extent that Iran now appears to be self-sufficient and self-contained in its quest to become a nuclear armed nation," the letter said.

"We have already waited too long, Mr President. Now is not the time for platitudes regarding one of the greatest threats Israel has faced."

The letter calls on Obama to "articulate unwavering support for Israel".

"We cannot stress strongly enough the need to repair broken relations with Israeli leaders and to make our offer of support and assistance to Israel crystal clear if Israel finds it necessary to take action against Iran, in lieu of your reticence to do so," it said.

Last week, Obama warned against political opponents "beating the drums of war", without considering the human or economic cost of an attack on Iran. But that has apparently not discouraged the members of Congress who signed the letter.

Among the lead signatories are Trent Franks, a member of the House of Representatives armed services committee; Pete Sessions who has previously introduced legislation asserting that Israel has the right to attack Iran; and Joe Walsh, who last year submitted a resolution suggesting Israel has the right to annex the occupied territories if the Palestinians make a unilateral push for statehood.

The letter follows Obama's meeting last week with the Israeli prime minister, Binyamin Netanyahu, at which the president resisted pressure to commit to an early use of force against Iran if its nuclear programme crosses "red lines" in its development.

Obama last week told the influential pro-Israel lobby group, the American Israel Public Affairs Committee (Aipac), that "loose talk of war" and military bluster was helping Iran by forcing up oil prices – a warning apparently aimed at the Israeli leadership.

Aipac flooded Congress with its supporters to lobby in favour of a stronger stand by the president in support of Israel, including backing for resolutions that critics say appear close to authorisations for war.

However, support for Obama's policy has come from liberal pro-Israel groups such as Americans for Peace Now, a US offshoot of the Israeli group, Peace Now.

It described the president's position on Iran as "a valuable reminder from the leader of Israel's greatest ally that supporting Israel means supporting peace and sparing no effort to avoid war".

Americans for Peace Now has called on its supporters not to allow Aipac to portray itself as the sole voice of Jewish Americans when it lobbies Congress.

"Aipac and its activists are pressing members of Congress to support resolutions that effectively take all options off the table except military action, and that lower the bar for war," it said.

Another pro-Israel group, J Street, which was formed to challenge Aipac's influence, also came out in support of Obama's position that a conflict is not in the security interests of the US or Israel.

"The president's concern regarding the consequences of a military strike against Iran is shared by many in the American and Israeli security establishments," it said. "Sanctions are beginning to have an impact, and they should be given further time to take effect. A precipitous rush to military action against Iran does not serve the best interests of the United States, or of Israel."