GAZA CITY - Hamas authorities in the Gaza Strip executed three Palestinian men for murder on Tuesday, the attorney general said, drawing condemnation from the United Nations.

The three, who were not named and whose cases were unrelated, were put to death by firing squad behind closed doors, security sources said.

"To achieve public deterrence and curb crime, the competent authorities carried out at dawn on Tuesday execution rulings against three of those convicted of shocking murders," a statement from the attorney general said.

Rupert Colville, spokesman for the UN High Commissioner for Human Rights, condemned the executions which he said occurred "despite serious and widespread concerns that international fair trial standards were not respected."

In theory all execution orders in the Palestinian territories must be approved by president Mahmud Abbas, who is based in the occupied West Bank.

But Hamas that runs the Gaza Strip, no longer recognises his legitimacy, and Gaza attorney general Ismail Jaber recently announced that the authorities there would carry out the executions without Abbas's backing.

Hamas and Abbas's Palestinian Authority agreed a unity deal in April 2014 which was supposed to lead to a joint technocratic government, but the deal was never fully implemented and the two sides remain at loggerheads.

The UN statement said that the failure to get Abbas's backing for the executions represented a breach of Palestinian law.

Sari Bashi, Israel and Palestine director at Human Rights Watch, said Hamas authorities had been under pressure to act due to perceptions of increased crime in Gaza, including a number of grisly murders of civilians.

"This is an attempt by the government to show they are doing something," she told AFP, "but killing criminals is not going to decrease crime."

"These people were convicted in a judicial system where torture and coercion are common," she added.

The attorney general said the three men had been given a fair trial.

Jaber had initially said he wanted executions to be carried out in public. It was unclear if that would happen in the future.

At the time of the attorney general's announcement last week, 13 men, most convicted of murder connected to robberies, were awaiting execution.

The last public executions in Gaza were during the 2014 war with Israel when a firing squad from Hamas's armed wing shot dead six alleged collaborators with Israel in front of Gaza City's main mosque following prayers.

Mkhaimar Abusada, professor of political science at Gaza's Al-Azhar University, said the reintroduction of the death penalty would be popular in the Palestinian enclave.

"Gaza is very much a tribal society, meaning people believe in revenge for tribal or familial reasons," he told AFP.

"In order to avoid people taking the law into their own hands the best (route) is through government or the judicial system."

On the streets of Gaza, which has been blockaded by Israel for 10 years and has an unemployment rate of almost 45 percent, opinion about the executions was mixed.

"Whoever kills people should be killed as an example to others," said Nahed al-Luh, in his 60s.

But Amin Abed, 25, called the executions "illegal" because "Hamas didn't deal with the Palestinian Authority and the president didn't ratify the verdicts."

According to the Palestinian Centre for Human Rights (PCHR), nine death sentences were handed down in the Gaza Strip in 2015 and two in the occupied West Bank, run by the Palestinian Authority.

So far this year, around 10 more people have been sentenced to death in Gaza.

Palestinian law allows the death penalty for collaborators, murderers and drug traffickers.

Of the more than 170 Palestinians sentenced to death since the creation of the Palestinian Authority in 1994, around 30 have been executed, mostly in Gaza, according to the PCHR.