Morocco has accused Algeria of expelling 55 Syrians across the countries' shared border. Rabat condemned its neighbour's "inhumane behaviour" towards the migrants who included "women and children in a very vulnerable situation".
The Syrians were sent across the frontier near the desert town of Figuig, Morocco's interior ministry said in a statement late Friday.
In searing heat and rugged terrain, they had reached the area in several groups before being "surrounded" by Algerian police, according to the Moroccan authorities.
The expulsion was "contrary to the rules of good neighbourliness advocated by Morocco," it added.
An NGO official in Figuig, who requested anonymity, said the Syrians were still stuck at the border Saturday without access to water or food.
Moroccan media reported that the Syrians had been left to their fate in the border region as Morocco prevented them from entering its territory. The frontier has been closed since 1994.
The ministry's statement did not say whether they had been allowed to seek asylum in Morocco.
"This is not the first time that the Algerian authorities have expelled immigrants to Moroccan territory," it said.
A statement from the Moroccan foreign ministry late Saturday expressed "deep concern" over the situation, saying Algeria's ambassador to Rabat had been shown photographs showing that the Syrians had crossed from Algerian territory and then tried to enter Morocco.
"Algeria must meet its political and moral responsibility with regard to this situation," the ministry said, adding that "the humanitarian tragedy" faced by Syrian civilians should not be used "to sow trouble on the Moroccan-Algerian border".
In mid-March, a Moroccan migrant rights group, GADEM, reported that around 30 sub-Saharan migrants had been arrested in Morocco then left stranded in no man's land between Morocco and Algeria, having been deported from both countries.
Morocco adopted a new migration policy in 2013. In December it launched a new campaign to regularise the status of clandestine migrants in its territory, most of them from sub-Saharan Africa.
Rabat insists its migration policy is "humane and generous" -- in contrast, it says, with the policy of its Algerian rival.
In January 2014, Morocco summoned Algeria's ambassador to protest against its alleged expulsion of Syrians across their common border.
Algeria responded in kind, saying its border guards had merely refused to allow Syrians deported by Morocco to enter its territory.