RNA - Tung-Lai Margue, the European Union delegation chief, in a statement demanded the government to ensure perpetrators were prosecuted swiftly.
"It is important that the government and the police ensure that there is no impunity for hate crimes," Margue said.
Margue made the remarks after he visited the Dewatagaha mosque on Thursday in Sri Lanka's capital to express solidarity with Muslims in the wake of the attacks.
He was accompanied by top diplomats from Australia, Canada, the Netherlands, Norway, South Africa and Switzerland.
Sri Lankan law enforcement agencies have yet to make any arrests in connection with a series of arson attacks which targeted mosques, shops and a cemetery in recent weeks.
The attacks have been blamed on a hardline Buddhist leader who is now on the run.
Rights groups have accused Sri Lankan security forces of failing to stop a series of hate crimes and not prosecuting a controversial Buddhist monk who went into hiding last week.
Activists have warned that police inaction could spur communal unrest similar to anti-Muslim riots in 2014 that left four dead.
The riots in mid-2014 were widely seen as the catalyst that led to then president Mahinda Rajapakse's downfall in January 2015.
President Maithripala Sirisena had vowed to investigate anti-Muslim hate crimes after assuming power in 2015.
Muslims make up 10 percent of Sri Lanka's 21 million population. They have, however, emerged as kingmakers in a country where the majority Sinhala-Buddhist community is split down the middle between two national parties.
Muslims have called on the government to protect them from hate attacks by Buddhists.
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