US President Donald Trump has defended the US strikes on Iran in a letter to Republican Senator Chuck Grassley, adding that it is “not possible” at this time to know how long the US military operation will go on, CNN reports.

“Although the United States desires a quick and enduring peace, it is not possible at this time to know the full scope and duration of military operations that may be necessary,” Trump wrote in yesterday’s letter, which CNN obtained.

The report added that Trump pointed to the US’s attempts to negotiate “a diplomatic solution” with Iran but insisted that “the threat to the United States and its allies and partners became untenable”.

Trump also termed the strikes as “collective self-defence of our regional allies, including Israel”.