A series of terror bombings in Pakistan's sensitive northwest border area near Afghanistan appears to be part of a new campaign by the Soviet-backed government in Kabul to escalate political tensions surrounding the presence here of millions of Afghan refugees.

The bombings have coincided with the rise to power in Afghanistan of Najibullah, the former head of the country's secret police who assumed control of the communist party leadership 10 weeks ago from the ailing Babrak Karmal. Najibullah, who is considered by diplomats and political observers to be more inclined to use such methods than his predecessor, is believed by a number of these sources to have inspired the bombings to intimidate Pakistan, which supports anti-Soviet Afghan resistance fighters, or mujaheddin.

Two explosions last weekend blew up a hotel run by mujaheddin, and a tractor and wagon carrying Afghan refugees, in the Kurram district southwest of here. More than 20 Afghans have died so far from those blasts.

During the past month, explosions also have derailed two passenger trains, injured passengers on public buses, killed vendors and passers-by in the bazaars of Peshawar's old city and destroyed highway bridges and electrical transmission lines. Press accounts during the past month have reported at least 25 deaths and nearly 100 people injured in explosions in the North West Frontier Province.

Police have reported the use of various types of bombs, made of dynamite or plastic explosives, as well as land mines and rockets in the explosions. Police, who are tightening security in Peshawar, say they have arrested both Afghans and Pakistanis in connection with recent blasts.

While one Pakistani official who asked not be identified said there was a possibility that Pakistani leftists are involved in planting bombs, both foreign and local observers said the Afghan secret police, Khad, is responsible for many or all of the recent bombings.

Pakistani Prime Minister Mohammed Khan Junejo said the rise in border attacks is "an organized campaign" that has increased notably since Najibullah came to power. In an interview with Washington Post editors and reporters, Junejo said air attacks have "gone into the hundreds" and ground attacks have also increased.

Both official sources and other political observers said the Najibullah government has intensified bomb attacks in North West Frontier Province and border violations, although the government has not published comprehensive figures on such incidents since Najibullah became communist party leader.

Peshawar-based leftist party leader Khan Abdul Wali Khan, who, with other leftists, argues that Pakistan should negotiate a solution to the Afghan war directly with Afghanistan, implicity agrees that Kabul is behind the bombings. But he added that they are the natural outcome of Pakistan's joining the United States in helping the mujaheddin.

In speeches and interviews, Wali Khan has repeated that "if we send them bullets," the Afghan government "will not send us flowers."

In an interview last week with the Islamabad daily The Muslim, Najibullah insisted that Afghanistan has no role in the bombings.

"Social process and internal tensions . . . in Pakistan are all due to inner factors and motives of the Pakistani society," he said, adding that accusations of Afghan involvement were intended "to deceive the world."

Najibullah repeated accusations that Pakistani and U.S. support alone is sustaining the 6 1/2-year-old guerrilla war against Soviet and Afghan government forces.

"We refuse to accept that a counterrevolution has any base in Afghanistan except the sustenance it gets from outside via Pakistan," he said. "We have no doubt that such efforts by Pakistan will lead to the burning of their own fingers."

Violence is not unusual in Pakistan's northwest border region, where the government exercises minimal control over Pathan tribes that have feuded regularly in sectarian and land disputes. But political observers here said recent attacks in Peshawar and other towns have had some success in what they view as Soviet and Afghan government objectives to make the presence of Afghan refugees, and therefore Pakistan's Afghan policy, a politically contentious issue.

In recent weeks, two Peshawar citizen groups have emerged to issue press releases and paint slogans on city walls calling for better security. One organization, calling itself the Peshawar Citizens' Front, has demanded that Afghan refugees who hold jobs or run businesses within the city be forced back into the refugee camps.

The Afghan refugee presence in Pakistan, described by U.N. officials as the world's largest single refugee population, is estimated at anywhere from 2 million to more than 3 million. Despite the refugees' economic competition for jobs, and even grazing lands for sheep, their relations with Pakistanis have remained, in the words of one western aid official, "Much better than one would have guessed."

Observers disagree on whether bombings and border violations may be seriously eroding that relationship. But one local journalist, Rahimullah Yusufzai, said, "Now you can hear a very few public calls for expulsion of the refugees to Afghanistan . There was a time when this was not said openly." Washington Post staff writer Don Oberdorfer contributed to this report.