The kidnapping of a top Jordanian diplomat in Beirut a week ago and a recently uncovered plot to assassinate Jordanian Prime Minister Mudar Badran have raised grave concern here that Syria, which is blamed for both, has decided on a full-fledged terrorist campaign to destabilize the kingdom of King Hussein.

There are no signs yet the two countries are moving toward open warfare. But Jordan is bracing itself for the worst in terms of Syrian attempts at war by other means, namely sabotage and subversion.

"The Jordanians feel there is no going back and that [Syrian President Hafez] Assad is dedicated to the destruction of the regime," said one Western diplomat.

The mood here is one of depression and even a measure of helplessness among Jordanian authorities about the deteriorating situation, since Jordan does not want to resort to similar tactics in retaliation and is no match alone in an open military confrontation with the far larger Syrian armed forces.

So far, the Jordanian government is reacting cautiously pending confirmation of the fate of its kidnaped charge d'affaires, who was reported by a Lebanese Christian radio station to have been executed.

It has tightened internal security, with more police road checks and patrols than normal here in the streets of the capital. Surveillance of Syrian nationals entering the country also has been increased.

But the thrust of Jordan's reaction to date has been to try to mobilize Arab, Asian and Western diplomatic pressure on Syria to release the diplomat and halt what Jordanians fear is becoming a concerted Syrian drive to undermine the government.

Jordanian officials profess puzzlement over the motives of Syria's

Jordanian officials profess puzzlement over the motives of Syria's increasing hostility but suggest Assad is seeking a foreign scapegoat for his deepening domestic political troubles. Western diplomatic sources believe Syria is also trying to undermine the king's bid to play the role of broker in the next stage of Middle East peace negotiations.

The Syrian-Jordanian feud has been further aggravated by Jordan's all-out support of Iraq in its war with Iran, while Syria views Iraq as an enemy and has taken the Iranian side.

"We seem to have moved through the classic phases in the escalation of hostility and a propaganda war," remarked Crown Prince Hassan Ibn Talal as he traced in an interview the history of souring Jordanian-Syrian relations over the past eight months.

Now we seem to be in a third phase of this scenario which is basically sabotage," he said. "I don't know really where this path can take us."

The war of words between Jordan and Syria began last summer with Syrian charges that the government here was providing military aid and comfort to the Moslem Brotherhood, the group leading the current armed opposition to the Assad government. Jordan has denied the charges repeatedly, but Syria remains unconvinced.

Then, last November, Syria moved 1,000 tanks and 50,000 troops to the Jordanian border, touching off fears of an incursion into this country, although the main Syrian objective was apparently to disrupt the Arab summit conference being held here.

Saudi Arabia stepped in to defuse the November crisis successfully. But neither side seems interested in Arab mediation this time.

Early this month, even before the Beirut kidnapping, Prime Minister Badran told the Jordanian National Consultative Council that the dispute between Jordan and Syria was too serious now to be "ended by mediation . . . or a mere handshake."

In the same speech, he said the Syrian "conspiracy" against Jordan "is now taking on another form of despicable plotting which is undertaken only by cowards who operate under the cover of darkness."

Badran did not elaborate further but he may have been referring to Jordanian evidence of a Syrian attempt on his life.

While there has been much publicity about the kidnapping of the Jordanian charge d'affaires in Beirut, Hisham Moheisen, by an apparently pro-Syrian terrorist group, little has been divulged so far about the apparently even more serious incident involving an attempt to assassinate the prime minister.

Shortly before Moheisen's disappearance, Jordanian security forces intercepted a team of Syrian commandos that had infiltrated the country apparently with the mission to kill Badran, according to Western diplomatic sources.

The only report of the incident published here so far, in the Engligh-language Jordan Times, said the commando group consisted of about a dozen men and was led by a Syrian Army colonel. There was no precise indication when the team was captured, but the newspaper said it was "a few days" before the kidnaping.

Western sources said the Syrians might have targeted Badran because he was Jordan's former security chief and is suspected in Damascus of having sympathies for the Moslem Brotherhood, which in this country is a legal organization and is allowed to operate freely.

Jordan had denied any involvement with the Brotherhood's own campaign of terrorism inside Syria to bring down the Assad government. Last summer and fall, Jordan cooperated with Syrian intelligence in tracking down a number of individuals living in this country and suspected of belonging to the Moslem fundamentalist group operating in Syria.

Jordanian sources say the government even opened its files to Syrian intelligence agents who were seeking the repatriation of 17 suspected Brotherhood members. At least two, and possibly four, were handed over to the Syrians, but the Jordanians were appalled to learn they then were executed.

"You do not deliver people to their[WORD ILLEGIBLE] remarked one Jordanian official.

The Syrian action helps to explain why Jordan is now refusing to hand over two Syrian pilots who defected last fall and flew their Mig fighter jets to the Amman airport. Five others are reported to have defected to Iraq, also with their aircraft.

Jordan immediately returned the two aircraft. Western diplomatic sources are not sure the pilots are in the country now, and say they may have been sent to Iraq.

The group claiming responsibility for the kidnaping of the Jordanian charge in Beriut, the Eagles of the Revolution, asked for the return of the pilots in exchange for Moheisen's life.

However, another theory circulating here is that the kidnaping may have been in retaliation for Jordan's interception of the Syrian commando group and its release may be the real objective of the Beirut operation.

Whether there are any secret talks under way between the two governments over these two incidents was not immediately known. But given the present tense and distrustful atmosphere hanging over Jordanian-Syrian relations, it would appear that such talks would be extremely difficult.

Right now, the Jordanians are waiting to see just what Syria intends to do next.

"The situation is really as serious as the Syrians want to make it," said Crown Prince Hassan, reflecting the Jordanian sense of resignation before the renewed crisis.

"There is nothing we can do," said a government spokesman. "There is a stalemate and an escalation in the war of words and we hope nothing more."