Former U.S. Ambassador to Israel Samuel Lewis on Saturday warned of a dangerous step-up in Middle East tensions if Syria deploys new surface-to-surface missiles it reportedly is buying from China.

"I think one of the most worrisome things about the Middle East for the next three or four years is this rising technology level and particularly missile technology," said Lewis, the ambassador to Israel from 1977-85, on Israel radio."It raises the stakes a great deal, and the next time tensions rise - unless we can get a peace process relaunched - I think this is a period of some danger in the region."

Newsweek magazine quoted an unidentified Reagan administration official last week as saying China planned to sell Syria its newest M-9 missiles, which have a range of 372 miles.

Syria can already hit most of Israel's territory with its Soviet-made Frog-7, SS-1 and SS-21 surface-to-surface missiles that have a range of 37, 160 and 62 miles respectively.

But Israeli reports said the M-9 missiles could be placed deeper inside Syrian territory, making retaliation more difficult.

"I can certainly understand why the prospect of a sale of a new missile to Syria would cause great worries here," Lewis said.

"I'm sure that it will be, if it is not already, a matter of serious discussion between the United States and China . . . but how much real effect we would have on Chinese decisions I'm very skeptical."

Prime Minister Yitzhak Shamir's top aide, Yossi Ben-Aharon, said on Israel radio he had no confirmation that Syria purchased the M-9s. But he said Israel was deeply concerned about any missiles Syria possesses.

"Now you know the Syrians have missiles of their own from the Russians . . . and they have developed a chemical warfare capability," Ben-Aharon said.

"In other words, there is a source of threat, very formidable. We are concerned to a very large extent over these developments."

Defense Minister Yitzhak Rabin is visiting the United States and is expected to raise the Chinese missile sale issue with U.S. officials.

Ben-Aharon said Israel's response to the reported sale should be "military and political and diplomatic."

"With regard to the military, we have to exercise, to invest maximum effort in order to develop the kind of countermeasures that would neutralize this threat," he said.

Ben-Aharon did not elaborate. However, army chief of staff Lt. Gen. Dan Shomron warned last week that Israel would respond to the deployment of Syrian missiles in various ways, including attacks on missile sites.