Amnesty International on Wednesday urged Jordan to keep its border open to refugees fleeing Syria after a deadly frontier attack prompted Amman to declare the area a "military zone".
King Abdullah II vowed to hit back with an "iron fist" after Tuesday's suicide bombing that killed seven Jordanian soldiers near the Syrian border and a makeshift refugee camp.
The Jordanian army said the suicide bomber set off from the camp in no-man's land and blew himself up at the military outpost.
The military declared the remote desert regions that stretch northeast to Syria and east to Iraq "closed military zones".
"We will deal firmly with any vehicle of individual that moves in the area without (prior authorisation) because they will be considered enemy targets," it warned, without explicitly saying if the border with Syria would be closed.
Amnesty said Jordan's response "must not descend into closing the border and denying humanitarian aid to tens of thousands of Syrian refugees fleeing armed conflict."
"A total closure of the border and denial of humanitarian aid to the area would inevitably lead to extreme hardship among those unable to find refuge and put their lives at risk," it added.
Jordan hosts hundreds of thousands of Syrian refugees and thousands more have been stranded at the frontier since January.
Amman insists newcomers must be screened before entering Jordan to ensure they are genuine refugees and not jihadists from the Islamic State group or Al-Qaeda trying to infiltrate the country.
"Jordan has a duty to protect civilians from armed attacks but its security measures must not violate its international legal obligations to provide protection and assistance to refugees who are desperately fleeing the very same type of violence," said Amnesty's Sherif Elsayed Ali.
"Denying them entry into Jordan amounts to a violation of international law," added the head of refugee and migrant rights at the London-based watchdog.
There was no immediate claim of responsibility but Jordan is part of the US-led coalition fighting IS in Syria and Iraq, and has been targeted by IS jihadists before.
Jordan also hosts coalition troops on its territory.
On Wednesday King Abdullah urged the coalition to intensify the battle against IS and "eliminate this terrorist group" during talks in Amman with US special envoy Brett McGurk.
UN High Commissioner for Refugees Filipo Grandi also condemned Tuesday's attack, the UNHCR said in a statement Wednesday.
"UNHCR mourns the loss of life from this outrage," Grandi said.