US senators who crafted tough new sanctions legislation targeting Iran urged their colleagues on Tuesday to resist White House efforts to water it down ahead of a final vote expected this month.

Democratic Senator Robert Menendez and Republican Senator Mark Kirk argued in a letter that their proposal -- approved in a rare 100-0 Senate vote last week -- "is tough, responsible and, most importantly, bipartisan."

The sanctions measure is part of an annual military spending bill now subject to negotiations between the Senate and House of Representatives to blend their rival versions into a final compromise both chambers can approve.

Menendez and Kirk wrote to House negotiators to urge them to resist a push by US President Barack Obama's administration to weaken their legislation, which aims to force Iran to halt what the West charges is a nuclear weapons program.

The measure "provides the Administration another key tool to curb Iran's pursuit of nuclear weapons while keeping oil markets stable and encouraging other nations to reduce Iranian oil purchases," they wrote.

"With the support of every single United States Senator, it needs no alterations," said the senators, whose proposal aims to cut Iran's central bank off from the global financial system.

The measure calls for freezing the US-based assets of financial institutions that do business with the central bank.

It would apply to non-US central banks that do so for the purpose of buying or selling petroleum -- Iran's chief source of revenues -- or related goods, amid growing fears that time is running short to solve the standoff peacefully.

The Obama administration has reportedly sought to lengthen the time between enactment of the underlying bill and the imposition of sanctions.

US officials have warned that depriving global markets of Iranian exports could send oil prices sharply higher, handing Tehran a windfall at a time when it has struggled to cope with painful international economic sanctions.

To address that, Kirk and Menendez's measure says the sanctions would only apply if Obama determines that there is sufficient oil from other producers to avoid disrupting global markets, and enables him to delay them if he determines that to be vital to US national security interests.

And Kirk said this week that Saudi Arabia's ambassador had told him of the kingdom's "great willingness" to boost its output to meet any resulting shortfall in supply.

The sanctions would not apply to sales of food, medicine and medical devices.