Evidence is growing that Russia is behind a significant increase in the use of cluster bombs in Syria, campaigners said Thursday.

A coalition of NGOs led by Human Rights Watch (HRW) said in an annual study that more than 400 people were killed or maimed by the banned munitions in the world last year.

They linked the increased use of cluster bombs in Syria to Russian forces who are carrying out air strikes in support of President Bashar al-Assad's regime.

"Since Russia began its joint operation with Syrian forces at the end last September, we have seen an increase in the number of cluster munition attacks on opposition-held areas," Mary Wareham, HRW's arms advocacy director and editor of the report, told a press conference.

"And at the moment we see evidence of cluster munition attacks every week, if not almost every day, which is highly disturbing," Wareham added.

Russia has repeatedly denied using cluster bombs, which spray bomblets indiscriminately.

HRW admitted it was difficult to determine whether it was specifically Russian or Syrian forces which had used the bombs.

"Nonetheless, this is a joint military operation, so collectively together they are responsible for the actions of their coalition," Wareham said.

A total of 248 people were killed or injured by the munitions in Syria last year, almost all civilians, the Cluster Munition Monitor report said.

The bombs also killed or maimed 104 people in Yemen in 2015.

The study provides an overview of how countries are implementing a landmark 2008 convention which bans all use, production, transfer and stockpiling of the weapons.

Syria and Russia are not among the 100 signatories of the convention, but HRW says they remain bound by international law, which bans the indiscriminate attacks that are the hallmark of cluster bombings.

- Russian offensive -

Attacks in Syria using the illegal weapons fell considerably in late 2014 and early 2015, but rose again after Russia began supporting Assad's forces last year, HRW and its partners said.

In the four years since the Syrian regime launched its first air strikes in July 2012, at least 360 cluster munition attacks have been recorded, with 76 since Russia's intervention began last September.

"The actual number is likely far higher," the report said.

The report, which was co-authored by several groups including Handicap International, said there was "compelling evidence" that cluster bombs had been used "on opposition-held areas of governorates such as Aleppo, Homs and Idlib, and on armed opposition groups".

- Soviet-era bombs -

Syrian government forces have used at least 13 different types of cluster munitions produced by Russia and Egypt and some dated from the Soviet era, the report said.

Several of the cluster bombs dropped since Russian forces entered the Syrian war last year were produced in 1989-91, it added.

The report said this appeared to be "a noticeable shift" from before the Russian intervention, "when production markings on the cluster bombs used in Syria showed they were produced in the 1970s and 1980s."

A total of 417 people fell victim to the munitions worldwide last year, with children making up 43 percent of those killed or injured.

Yemen is the only other country where cluster bomb attacks have been documented since July 2015.

The report said the Saudi-led coalition waging a military campaign in support of Yemeni President Abedrabbo Mansour Hadi's embattled government carried out at least 19 cluster munition attacks between April 2015 and last February.

Saudi Arabia has denied using cluster munitions against civilian targets.

"The best way to ensure that cluster munitions don't harm civilians in Syria and Yemen is to stigmatise their use and press countries that are using them to stop the attacks," Wareham said.

Syria and Yemen dominate the statistics, but six other countries recorded casualties, mainly from unexploded submunitions from attacks that took place years ago.

Many of these devices fail to explode on impact, meaning countries such as Cambodia, Iraq, Laos and Vietnam often find it impossible to clear what become de facto landmines.

Furthermore, many bomblets are brightly coloured, attracting children and exploding when they are picked up.

In the United States, Textron -- the last US maker of cluster bombs -- this week announced it would cease producing the weapons, citing reduced demand and difficulties securing approvals for foreign sales.