JUBA, June 26 (Reuters) - Fighting between government forces and armed groups around a town in the northwest of South Sudan has forced thousands to flee, the United Nations and an aid agency said on Saturday.

The clashes around Wau, northwest of the capital, are the latest flare-up in a nation which was mired in conflict for more than two years until a peace deal was reached between President Salva Kiir and rebel leader Riek Machar, now vice president.

The peace deal was signed last year but took many months to implement. Machar returned to Juba in April.

The latest fighting was between forces of the ruling Sudan People's Liberation Army and armed groups, according to a statement by U.N. Secretary General Ban Ki-moon which was issued to condemn the clashes and to call for talks.

It was not clear who the armed groups were. There was no immediate comment from the government.

The secretary-general "urges all parties to agree to dialogue to resolve their political disputes," the statement said. "He regrets the reported loss of lives."

He also praised the U.N. mission in South Sudan, UNMISS, for "taking pro-active steps to protect fleeing civilians outside their base in Wau". UNMISS was deploying more resources to the area, the statement added.

The International Committee of the Red Cross said in a statement that thousands of civilians had been forced to flee and said it was "seriously concerned by the immediate humanitarian consequences".

(Reporting by Denis Dumo in Juba, George Obulutsa in Nairobi and Stephanie Ulmer-Nebehay in Geneva; Writing by Edmund Blair; Editing by Digby Lidstone)