A global outcry has followed the killing of Dutch Jesuit priest Frans van der Lugt, who was shot dead by masked gunman in the besieged city of Homs.

The 75-year-old priest lived in Syria for decades and gained prominence for his insistence on staying in the city, a rebel stronghold which had been blocked for nearly two years by forces loyal to Syrian president Bashar al-Assad.

Van der Lugt was shot dead by a single gunman in his monastery in the city's Bustan al-Diwan neighbourhood, from which he sought to raise awareness of the suffering of civilians in the blockaded city.

UN Secretary Ban Ki-moon condemned the murder, calling it an "inhumane act of violence". Ban said van der Lugt "heroically stood by the people of Syria" through growing difficulties in the country.

Vatican spokesman Federico Lombardi said van der Lugt showed bravery in remaining loyal to Syrian people despite an extremely dangerous situation. He praised the Jesuit father as a "man of peace" and expressed "great pain" over his death.

US state department spokeswoman Jen Psaki said Washington is "saddened" by the death of van der Lugt.

The priest stayed in Homs even as 1,400 people were evacuated during a UN-supervised operation that started on 7 February, which brought limited supplies of food into the city.

"Hunger defeated us! We can see its signs drawn over the faces," Van der Lugt wrote on a Syrian Christian Facebook page as the siege worsened.

"People are wandering the streets screaming; We are starving, we need food!," he wrote in a statement published in English and French. "We are living a scary reality."

The Syrian government's official news agency Sana blamed the priest's death on "terrorists", a term used to describe rebels who are opposed to Assad rule.

The office of Ahmad Jarba, the president of the opposition National Coalition, condemned the murder "in the strongest terms."

It said the Assad regime was "ultimately responsible for this crime, as the only beneficiary of [van der Lugt's] death."