* Cleric released after four hours of questioning - lawyer
* U.S. accuses him of funding militants, on sanctions list
* Lawyer says cleric plans to bring lawsuit against U.S. (Updates with cleric's release)
KUWAIT, Aug 18 (Reuters) - Kuwait briefly detained a prominent Sunni Muslim cleric less than two weeks after the United States included him on a sanctions list on suspicion he was funnelling money to militants in Iraq and Syria, his lawyer said on Monday.
Shafi al-Ajmi, who was detained on Sunday on the border with Saudi Arabia while returning home from a pilgrimage, was released after four hours of questioning, his lawyer said.
"He was released without any charges," the lawyer, Mohammed al-Jumia, told Reuters. "He is now at home."
Jumia said he planned to file a lawsuit against the U.S. Treasury to lift his client's name from its blacklist, but gave no further details.
An Interior Ministry spokesman could not immediately be reached for comment.
Kuwait has been one of the biggest humanitarian donors to Syrian refugees through the United Nations, but it has also struggled to control unofficial fundraising for opposition groups in Syria by private individuals.
The government of the U.S.-allied Gulf Arab state has stepped up its monitoring of individuals and charities suspected of collecting donations for militants linked to al Qaeda in Syria and in Iraq.
On Aug. 6, the United States imposed sanctions on Shafi and two other men suspected of funnelling money from Kuwait to Islamic State, the al Qaeda splinter group that has seized swathes of territory in Iraq and Syria. Washington also said the men had helped smuggle fighters to Afghanistan and Iraq.
Last year, Kuwait banned a television show that Shafi appeared on, saying it incited hatred.
As well as calling for armed opposition to Syrian President Bashar al-Assad, Shafi has called on supporters to torture and kill fighters in Syria linked to the Lebanese Shi'ite group Hezbollah, evidence of how the Syria conflict has aggravated Sunni-Shi'ite tensions across the Middle East.
Kuwaiti Information Minister Sheikh Salman al-Humoud al-Sabah said at the time that authorities would investigate how a show featuring the cleric was allowed to be broadcast on state television.
On Friday, the U.N. Security Council imposed sanctions on six people suspected of financing Islamist militants, including two Kuwaitis, in a move aimed at weakening Islamic State and al Qaeda's Syrian wing, Nusra Front.
One of the sanctioned Kuwaitis was identified as Hajjaj al-Ajmi, another prominent Muslim cleric who belongs to the same extended tribe as Shafi al-Ajmi's. (Reporting by Ahmed Hagagy and Sami Aboudi; Editing by Yara Bayoumy and Alison Williams)