The assailants fired for several minutes in broad daylight at the church in Karak governorate on Friday, and also wounded 40 people in the rare attack on Jordan's Christian minority.
Security was stepped up at malls, foreign embassies and other locations across the country, the US embassy in Amman said.
A priest described how he and others ordered people to get to the ground at Saint George's Greek Orthodox church, about 120km southwest of the capital Amman.
"A sense of panic took hold among those present," Theodoros Soueiss told Reuters.
"The gunfire was indiscriminate, and you never knew where a bullet might land."
King Abdullah condemned the "despicable terrorist act" as he chaired a National Security Council meeting and said there was no room for leniency when it comes to the lives of Jordanians, the royal court said.
Such attacks are highly unusual in Jordan, a relatively stable state in the Middle East, although authorities there say they have foiled several Islamist plots in recent years.
There was no immediate claim of responsibility for the attack in the kingdom, a major partner with the US in the fight against militant groups.
Officials have said it is too early to determine whether the attackers had links to known groups inside or outside Jordan.
"A massacre, a massacre in every sense of the word," said Tarek Ammarin at a hospital where the wounded were being treated.
A relative of Ammarin's was injured in the attack.
Christians make up a minority of Jordan's population.
The country is home to several biblical sites that attract Christian pilgrims from around the world.
Bahjat al-Rushda, mayor of Shihan village in Karak, joined a long list of officials condemning what they called an attack by extremist terrorists.
"God willing, this country will remain secure," he said.
