EU foreign policy chief Federica Mogherini condemned a deadly attack on a Jerusalem synagogue by Palestinians Tuesday as an "act of terror" and urged regional leaders to try to calm the situation.

"The attack in the Har Nof synagogue this morning killing at least four people can only harm any step forward towards peace. It is an act of terror against worshippers at morning prayers and is condemnable by all means," Mogherini said in a statement.

Mogherini -- who called for the establishment of a Palestinian state during a visit to Gaza earlier this month -- warned against the risk of a cycle of violence if all sides did not try to ease the tensions.

"I call on all leaders in the region to work together and do their utmost to immediately calm down the situation and prevent further escalation," she said, adding that all sides should "refrain from any action that would worsen the situation by way of incitement, provocation, use of force or retaliation."

The former Italian foreign minister implicitly criticised the Islamist Hamas movement, which hailed the attack on the synagogue, saying: "I utterly condemn all statements calling for or praising such attacks."

But she also pushed for progress in the peace process, adding: "The lack of progress towards the two-state solution will systematically ensure the next round of violence."

On Monday, Mogherini had deplored Israeli plans to build new settlements but denied reports that the 28-nation European Union planned sanctions against Israel as a result.