UNITED NATIONS, April 12 (Reuters) - U.N. Secretary-General Ban Ki-moon called on Thursday for the immediate release of a BBC journalist, abducted in the Gaza Strip a month ago, and offered to help secure his freedom.

The correspondent, Alan Johnston, has been held longer than any other foreign journalist seized in the Gaza Strip. There has been no claim of responsibility for the kidnapping.

"I sincerely hope that those who are responsible for this abduction should release him unconditionally and immediately," Ban told reporters. "I am deeply concerned. I will do whatever I can in my capacity as secretary-general."

Palestinian President Mahmoud Abbas said he had credible evidence that Johnston was "safe and well," according to the the British Broadcasting Corporation's director-general, Mark Thompson, who spoke to reporters after meeting Abbas.

Complimenting Johnston, Ban said his coverage of "Palestinian issues has earned a great reputation worldwide."

"Freedom of coverage, as well as freedom of the press, should be protected as a matter of principle," Ban said.

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