(CNN) -- Four tourists were killed and three injured Sunday when a bomb exploded in the Yemeni city of Al Shibam, a journalist told CNN.
The explosion occurred about 7:30 p.m. (12:30 p.m. ET), said Hakim Al-Masmari, editor-in-chief of the English-language Daily Yemen Post, who told CNN he got the information from his correspondents at the scene.
The bomb was very powerful and could be heard more than 2 miles away from the site, in the Al-Qubah District of Al Shibam, which is located in Hadramot province, Al-Masmari said.
The four people killed are South Korean citizens, he said. The nationalities of the three injured people were not immediately known.
The tourists were taking pictures of Shibam at sunset, Al-Masmari said. The old walled city is listed as a world heritage site by the United Nations Educational, Scientific and Cultural Organization; sunset is the best time to take pictures of it, he said.
A terrorist act cannot be ruled out, a security source said. There was no immediate claim of responsibility.
The South Korean victims were taken to a hospital, but had died at the scene, Al-Masmari said. Of the injured, at least one of them was in serious condition, he said.
In January, two female Belgian tourists and a Yemeni driver were killed by gunmen while the three were traveling in a convoy in Shibam, approximately 2.5 miles from the site of Sunday's bombing, said Al-Masmari
CNN's Caroline Faraj contributed to this report.