KABUL (Reuters) - Afghan President Hamid Karzai will discuss security, fighting terrorism and improving economic ties during talks with his Iranian and Pakistani counterparts, the foreign ministry said on Sunday as he left for regional talks.

Iran and Pakistan border Afghanistan and have a large stake in its stability because the impact of a flourishing drugs trade and decades of violence have often spilled across borders.

Washington is also pushing a more regional approach to tackling the growing strength of Taliban-linked militants across the south and east of Afghanistan.

In March, the three countries agreed to meet monthly to discuss efforts to fight opium production and drug smuggling and stabilize Afghanistan through increased economic cooperation.

Karzai, accompanied by his foreign minister, Rangin Dadfar Spanta, will meet with Iranian President Mahmoud Ahmadinejad and Pakistani President Asif Ali Zardari in Tehran.

The three countries will discuss "eliminating extremism, terrorism and drugs, which oppose the beliefs and ideals of Islam and are against the cultures of all three Islamic countries," the Afghan Foreign Ministry said in a statement.

Iran is a main route for the trafficking of drugs from Afghanistan, the world's top producer of heroin, and is also home to some 3 million Afghan refugees.

Tehran is also a key investor in Afghanistan's reconstruction, and in the past has raised concerns about the presence of U.S.-led troops in the country.

Pakistan, a vital ally of the United States as it struggles to bring stability to neighboring Afghanistan, is fighting Taliban militants on its own ground and launched an offensive this month in the Swat valley in the northwest.

U.S. President Barack Obama's strategy review for Afghanistan released in April indicated Washington's interest in engaging with Iran and placed strong emphasis on ensuring that Pakistan increases military pressure on its own Taliban-led insurgency.

(Reporting by Golnar Motevalli; Editing by Emma Graham-Harrison and Paul Tait)