Karzai trip may help Iran join US-led Afghan talks.
The United States, other world powers and neighbours of Afghanistan meet later this month to explore ways of restoring stability to a country where international forces are battling a resurgent Taliban insurgency.
In what is being seen as an overture by President Barack Obama's administration towards arch-foe Iran, US Secretary of State Hillary Clinton has said Tehran will receive an invitation to the gathering.
Karzai and his Pakistani counterpart Asif Ali Zardari will meet President Mahmoud Ahmadinejad ahead of a Tehran summit on Wednesday of the regional Economic Cooperation Organisation (ECO).
Although supreme leader Ayatollah Ali Khamenei has Iran's final say on issues of strategy, the meeting of the three presidents is expected to see Karzai urge Iran to accept the invitation to the March 31 talks.
"Karzai will push Iranians to attend this conference as it will reinforce his own position and also help to secure support in stabilising the situation in Afghanistan," Iranian political analyst Mashallah Shamsolvaezin told AFP.
A decision on whether Iran will attend is expected over the next few days.
Clinton did not say where the meeting would be held, but that Afghan and Pakistani officials would be invited, with NATO allies, donors, international organisations and "key regional and strategic" nations.
Karzai's trip gained added significance after his foreign minister, Rangeen Dadfar Spanta, and Pakistani Foreign Minister Shah Mehmood Qureshi both called on Monday for Tehran's help in supporting Afghanistan.
"We definitely want the support and cooperation of the Islamic Republic of Iran," Spanta told AFP.
Shiite Iran shares close ethnic and religious ties with Afghanistan and opposed the Sunni Taliban before the US assault on Afghanistan in the wake of the September 11, 2001 attacks.
Tehran is also concerned about drugs smuggling from Afghanistan to Iran.
Shamsolvaezin said if Iran attends the March 31 meeting and if the United States and Iran talk "it could have a positive influence from northern Afghanistan to south of Gaza," a region spanning Iraq, Lebanon, the Palestinian territories and Pakistan.
Iran has already developed close ties with neighbouring Shiite Iraq since its former foe Saddam Hussein's Sunni regime was toppled by US-led forces in 2003.
Its presence at the Afghanistan meeting would also be seen as acceptance of Tehran's influence in the region following attempts to isolate it because of its controversial nuclear programme.
The West, led by Washington, suspects the nuclear programme is aimed at making atomic weapons -- a charge Tehran strongly denies.
Since Obama took office, Washington has made overtures towards Tehran and even said it would be prepared to hold "face-to-face" talks within months. The two countries have had no diplomatic ties for three decades.
"Iranian attendance at the Afghanistan conference is one of the first rungs on the ladder to direct Iran-US negotiations, as it will be important to test goodwill and guarantees," Shamslovaezin said.