At least five people were killed by shelling or sniper fire this weekend in what one U.N. official on Sunday called the worst violence in the capital since a cease-fire took effect this year.

The city was placed on general alert after gunfire struck a plane carrying Yasushi Akashi, the top U.N. official in former Yugoslavia, Sunday morning.

Akashi, arriving in Sarajevo to seek renewed commitment to the Jan. 1 truce, appeared unperturbed. “Nobody can intimidate me with this kind of thing,” he said.

But he admitted he was not optimistic about extending the cease-fire beyond its May 1 expiration date.

“The parties have not excluded the military option (after May 1) and for that reason they are testing each other,” Akashi said.