Riyadh (AFP) - Military pressure may be needed to oust Syria's President Bashar al-Assad, US Secretary of State John Kerry said in Saudi Arabia on Thursday.

Survivors of the Boston marathon bombings choked back tears Wednesday as they told how a day of celebration turned into a horrific urban war zone of torn limbs and burnt flesh. Victims recalled smoke, chaos and terrified screams as they lay in excruciating pain fearing death from the deadly blasts near the finish line at the 2013 race. Karen McWatters, a mother of two who lost a leg, told how she held hands with Krystle Campbell, one of the three people who were killed, and how she felt life ebbing from her close friend. Despite agonizing injuries, McWatters said she pulled herself over burning pieces of shrapnel to get close to her friend.

The US ambassador to South Korea, Mark Lippert, was recovering from surgery Thursday after having his face and arm slashed by a knife-wielding activist in an attack applauded by North Korean state media. US Secretary of State John Kerry said Washington will never be deterred by "acts of violence" as the ambassador was rushed to hospital where his condition was described as stable after two-and-a-half hours of surgery that included 80 stitches to a deep gash on his right cheek. In a brief despatch, North Korea's official KCNA news agency called the attack "just punishment" and a valid "expression of resistance" that reflected South Korean public opposition to the joint drills which went ahead over Pyongyang's bitter protests.