JERUSALEM (CNN) -- The United States has expressed concern that Syria and NATO ally Turkey could come into conflict over Kurdish rebels and has urged both sides to exercise restraint.

"We have concerns about the possibility of this moving in a negative direction," said State Department spokesman James Rubin Monday after arriving in Jerusalem with Secretary of State Madeleine Albright for talks on an Israeli-Palestinian peace deal.

Turkish officials have accused Syria of allowing the country to be used as a base for Kurdish rebels who have been fighting for self-rule in southeastern Turkey.

The head of Turkey's influential military warned that differences with Damascus had reached a stage of "undeclared war" prompting worries that if clashes began, they might spread.

The newspaper Sabah quoted Gen. Huseyin Kivrikoglu, Turkey's military chief of staff, as saying: "There is a state of undeclared war between us and Syria. We are trying to be patient but that has a limit." Other papers had similar reports.

Syria, suspicious of Turkey's growing military ties with Israel, said this alliance was behind the rise in tension between the two neighbors.

Rubin said the United States had been in touch with both countries and had been urging restraint.

The United States has "expressed understanding in the past for Turkey's position on this matter, but let me be clear: We very much don't want this to go to the next step because in this case ... it would be a grave risk of a much larger conflict," Rubin said.

Israeli Prime Minister Benjamin Netanyahu said on Sunday that Israel is playing no part in the crisis.

In Washington, State Department spokesman James Foley said the United States had been talking to both governments about the issue and fully supported mediation efforts of Egyptian President Hosni Mubarak.

Ankara has sent thousands of troops to its border with Syria, prompting fears of war. There have been unconfirmed reports of Turkish jets violating Syria's airspace near the border.

Turkish President Suleyman Demirel warned Damascus on Sunday that Turkey would not allow its neighbor to continue sheltering Kurdish rebels. Syria denies the allegation.

In an attempt to find a diplomatic solution to the dispute, Mubarak met Syrian President Hafez al-Assad in Damascus on Sunday and will visit Ankara to pursue his mediation mission.

Turkish Foreign Minister Ismail Cem warned that time was running out for Damascus in the dispute.

"There is only one problem to sort out, Syria must end its support for terrorism...Turkey does not have the luxury or the time to be deceived by empty words that do not end in action," a Foreign Ministry statement quoted Cem as saying on Monday.

In apparent response, Syria called for Arab support on a level with that seen in the 1973 Middle East war when Arabs united behind Syria and Egypt against Israel.

"We affirm that the current challenges require the Arab nation to achieve real and effective solidarity like the one which we witnessed in October 1973," the official al-Thawra daily said.

The Arab League echoed Syrian and Iranian charges that Turkey's new informal ally Israel had fomented the row.

"Israel is behind this crisis," league Deputy Secretary- General Ahmed Bin Hilli said. "The aftershocks of the Israeli-Turkish pact are surfacing and are beginning to affect stability in the region," he said.

Lebanon, Jordan, Algeria, Saudi Arabia and Gulf countries all urged a diplomatic solution to the heightened tension between Syria and NATO member Turkey.

The Associated Press and Reuters contributed to this report.