GAZA, March 29 (Reuters) - Hamas security forces took $400,000 from a bank in the Gaza Strip on Monday, in a direct challenge to Palestinian authorities in the West Bank who had frozen the money to comply with money laundering regulations.

Hamas, which runs the Gaza Strip, said the men were executing a court order to seize the assets of a medical organisation, the Patient's Friend Association. Seeking to apply global regulations against money laundering, the Palestine Monetary Authority (PMA) froze the association's account after its board fell under Hamas control, banking sources said.

Hamas is defined as a terrorist organisation by Western powers. It seized control of the Gaza Strip in 2007 from forces loyal to Fatah, a party led by President Mahmoud Abbas. Abbas still controls the Ramallah-based Palestinian Authority.

The PMA described Monday's seizure, the first time Hamas had challenged the authority, as a sinful attack.

Ehab Al-Ghsain, spokesman for the Hamas-run Interior Ministry, said Monday's move was the implementation of a judicial decision. The association had resorted to court after the Fatah government froze its account in the bank, he said. One employee of the Bank of Palestine, who declined to be named because of the sensitivity of the situation, said the Hamas policemen had forced the staff to open the bank's vault and acted aggressively.

They took 1.5 million shekels ($400,000) and signed a paper showing the amount of money they had taken, the employee said.

It was the first time Hamas had challenged the PMA, which functions as regulator of the Palestinian banking system in the Gaza Strip and the West Bank.

This is a serious development. We are investigating the matter and then we will take the appropriate action, Jihad al-Wazir, the governor of the PMA, told Reuters. He declined to give details about the raid.

In a statement, the PMA called on Hamas to abide by the rule of law to safeguard the soundness of the banks so that they can keep providing services to the people.

Banks in the Gaza Strip would stage a strike on Tuesday to protest at the raid, Wazir said. Around a dozen banks, Palestinian- and Arab-owned, still function in the Gaza Strip, though their headquarters are in the West Bank.

Israeli banks severed ties with banks in the Gaza Strip in 2007 after the Jewish state declared it an enemy entity.

Hamas is hostile to Israel. It rejects interim peace deals between the Palestinians and Israel and is committed to armed struggle against the Jewish state. Gaza remains under an embargo enforced by both Israel and Egypt.