Palestinian President Mahmoud Abbas announced on Wednesday a day of mourning and ordered flags to be flown at half-mast to honor Ismail Haniyeh, the head of Hamas' political bureau, who was assassinated earlier that day.

Abbas condemned the assassination of Haniyeh as a "cowardly act" and a "serious escalation." 

He called on the Palestinian people and their factions to unite, demonstrate patience, and remain resolute in their struggle against Israeli occupation.

Hamas confirmed the death of Haniyeh in the Iranian capital, Tehran, early Wednesday morning. 

Iranian news sources reported that Haniyeh and one of his personal guards were killed in an attack on their residence in Tehran.

An investigation into the assassination is underway, according to Tasnim News Agency, with findings expected to be released soon. The Iranian Revolutionary Guard also stated that they are examining the circumstances surrounding Haniyeh's death.

Iranian media revealed that Haniyeh was killed by a missile launched from outside Iran, not from within Iranian territory. 

Tasnim also reported that the Iranian Parliament's National Security and Foreign Policy Committee will convene to discuss the assassination.

Haniyeh arrived in Tehran on Tuesday to attend the inauguration of Iran’s new president, Masoud Bezhkishian, at the Iranian Parliament. 

During his visit, he met with Bezhkishian and Iranian Supreme Leader Ayatollah Ali Khamenei.