Two Caritas workers and five of their family were killed in Mariupol, Ukraine, on Tuesday, the Catholic organization reported, adding that they likely perished a month ago.

“The tragic attack probably took place on March 15, when a tank fired shots at the building of the Caritas Centre in Mariupol, killing the two staff members and five of their relatives,” it said in a statement.

A report on the Vatican’s news portal, Vatican News, said the tank was Russian, citing unnamed “local Caritas sources”.

Russian troops have been battling for weeks to take control of strategically-located Mariupol, ahead of an anticipated massive onslaught across eastern Ukraine, as defending forces try desperately to hold them back.

Difficulties in communicating with those inside the besieged city have meant details of civilian casualties have been scarce and hard to verify.

Caritas said it was not able to definitively say what had happened, but it believed the two female staff members “and their family took refuge in the Centre during the shelling”.

“This dramatic news leaves the Caritas family horrified and shocked,” said Aloysius John, secretary general of Caritas Internationalis, slamming “the indiscriminate massacre of civilians”.

Russia is thought to be attempting to link seized Crimea with the Moscow-backed separatist areas of Donetsk and Lugansk in Donbas, and has encircled Mariupol, which previously housed over 400,000 people.