US President Barack Obama has sent across the sternest warning so far to Pakistan, asking it to stop using insurgents like Lashkar-e-Toiba (LeT) as strategic tool, according to a PTI report.

He has even warned of serious action if Pakistan doesn't stop using insurgents as a strategic tool, according to the report.

The report says that Obama has even written a letter to Pakistan President Asif Ali Zardari on the issue, which was hand-delivered by the US National Security Advisor during his visit to Pakistan.

In the letter, Obama has told Zardari that if it cannot deliver against terrorists, the US may be impelled to use "any means" at their disposal.

Reports suggest the message also includes an offer by Obama to try to "reduce tensions" between India and Pakistan.

Speaking to NDTV, the US ambassador Timothy Roemer, refused to comment on the letter but said Obama had extensive talks with PM Manmohan Singh on Pakistan, defeating terrorism and bringing Mumbai 26/11 attackers to justice.