Syria has sent more troops to the southern town of Deraa where activists say a deadly crackdown against pro-democracy protesters has entered a second day.

The developments in the volatile city, near the border with Jordan, come despite mounting international pressure on Bashar al-Assad's government to end the violence.

Security forces have shot dead 400 civilians since mid-March, Sawasiah, a Syrian human rights organisation, said on Tuesday. Up to 500 people had been arrested in the last two days, it said.

"We've been listening to live ammunition. Some snipers are working as well, but we don't know from where," a resident of Deraa told Al Jazeera on Tuesday.

"The snipers are on all the roofs. I'm now on my stomach, on the ground - I am really in a panicked situation. The city is quite in danger."