A Red Cross worker in Ukraine was killed Thursday after mortar shells hit near the office in the rebel-held city of Donetsk, the Associated Press reported.

The International Committee of the Red Cross said Laurent DuPasquier, a 38-year-old Swiss national, had been there for six weeks and was one of 20 who was working to support hospitals, refugees and families affected by the conflict.

DuPasquier had worked with the organization for more than five years in Pakistan, Yemen, Haiti, Egypt and Papua New Guinea.

"We are deeply shocked by this tragic loss," Dominik Stillhart, the Geneva-based organization's director of operations, said in a statement. "We understand that there were other civilian casualties in Donetsk today. Indiscriminate shelling of residential areas is unacceptable and violates international humanitarian law."

Officials do not know who is responsible for the shelling, which occurred during a cease-fire deal that began on Sept. 5, but appears to have first been breached on Wednesday.

Fighting has intensified since Wednesday where pro-Russian separatists near Donetsk's airport are trying to seize control of the facility from government forces, Radio Free Europe Radio Liberty reported.

There was shelling in the area the day before as well, killing three and injuring several others.

Aleksandr Zakharchenko, who calls himself the prime minister of the "Donetsk People's Republic," said Wednesday that separatists forces control "90 percent of the airport's territory" and intended on reaching 100 percent control in a few days.

A spokesman for U.N. Secretary-General Ban Ki-moon said in a statement, according to AP, that the shelling underscores "the fragility of the current cease-fire and the importance of ensuring a secure environment in south-eastern Ukraine that will allow humanitarian actors to carry out their work and deliver critical assistance to those most in need."

Meanwhile, authorities are unsure of where the shelling came from.

The rebels said it came from the Ukrainian side, according to AP, while Ukrainian Foreign Minister Pavlo Klimkin blames terrorists.