Two foreign journalists have been killed in Homs, activists say, as shelling of a district of the Syrian city continued amid warnings of an escalating humanitarian crisis.br br Omar Shakir, an activist in the city, told Al Jazeera that the deaths of Marie Colvin, a US reporter working for the UK's Sunday Times newspaper, and French photographer Remi Ochlik occurred as a building used by activists as a media centre was shelled on Wednesday.br br Al Jazeera's Laurence Lee reports.