"We support the efforts of Secretary of State Kerry," EU foreign affairs chief Catherine Ashton said at the close of a two-day meeting in Athens.
Kerry has laboured lately to bring Palestinians and Israelis back from the brink.
Yesterday he warned from Morocco that there were "limits" to the time and effort the United States could devote to the process "if the parties themselves are unwilling to take constructive steps".
But Palestinian president Mahmud Abbas has rejected Kerry's appeals to withdraw applications he signed on Tuesday to adhere to 15 international treaties in pursuit of recognition for the Palestinians' promised state.
"Kerry's initiative is without doubt in a critical phase," German Foreign Minister Frank-Walter Steinmeier said today.
"The radicals on both sides seem to start to prevail...We have to prevent that," Steinmeier told reporters.