BY A CORRESPONDENT

New Delhi: The evidence of serious deterioration in Indo-Pakistan relations mounts daily. However justified Indian Foreign Minister Swaran Singh's accusations that Pakistan is using - and escalating - tension in order to obscure the basic crisis in East Pakistan, there is no doubt that it is assuming the most serious proportions.

Another violation of airspace, another serious clash between Indian border security forces and West Pakistani troops on the East Pakistan border, and there could be war, if the pointers mean anything. This week UN Secretary General U Thant met ambassadors from both countries and mooted the idea of a security council meeting on the problem. Pakistan charged gross interference, India replied Islamabad had nothing but the Pakistanis' own "mismanagement and. . . oppression" to blame and referred tellingly to the 40,000-a day continuing influx of refugees.

Opinion in India is increasingly behind extreme measures. Defence Minister Jagjivan Ram was hard put to it last week to explain why Pakistani fighters which had allegedly violated Indian airspace in Kashmir were not shot down. He listed 43 previous violations and announced that in future India would open fire.

Even more alarming, while Prime Minister Indira Gandhi is exercising considerable restraint, important political forces are arguing that war with Pakistan would "cost less" than maintaining the refugees. They count on the present climate in Peking to assure noninterference by China. And the director of India's institute for defence studies has compiled a document which frighteningly concludes India has no alternative to opening hostilities-with the aim of capturing a slice of East Pakistan in which the refugees could be rehoused, and a provisional government of Bangla Desh set up under Indian protection.

He urges that it is not merely the problem of absorbing the refugees which faces India, but of continuing guerilla war in East Bengal, the consequences of which must be felt in West Bengal.

When India is likely to be dragged into war by mistake, the document argues, it would be better to open fire at a time of its own choosing. The report in the British press of an interview with Pakistani President Yahya Khan in which he allegedly threatened war with India and claimed Pakistan would not be alone in the struggle has done nothing to damp things down. To anger at what they see as international refusal to face the enormity of the burden India is bearing over the refugees, Indians also are feeling threatened.

Politicians express fears that over the years India has become friendless and isolated; the continuing supply of arms to Pakistan by the US appears to them the proof.

Mrs Gandhi is showered from all sides with demands to recognise Bangla Desh, withdraw India's ambassador to the US in protest, mount a blockade of East Pakistan to prevent military reinforcements and supplies reaching it - and even to alter policy on using India's new nuclear energy resources for military purposes. If India had the bomb, some argue, the world would be compelled to take its dilemma seriously.

These internal pressures are much aggravated by the financial crisis the refugees have brought on the country. Finance Minister Yeshwantrao Chavan will soon have to ask parliament for another massive grant for refugee relief, the Rs600 million (US$80 million) set aside in June having already been spent. The new figure could nearly double that and, whether it is included in the budget deficit or by fresh taxation, it imposes a terrible strain on a country with millions living below the subsistence line.

Mrs Gandhi came to power on a pledge to end poverty; but only Rs350 million ($46.6 million) is available for nutrition and employment schemes, and non-priority development is having to be sacrificed to feed what could conceivably amount to 10 million refugees before the year's end. The international appeal for $400 million, based on five million refugees who would return after six months, has only been three-eighths fulfilled.

India is bitter. But reports from East Pakistan suggest Yahya is facing increased guerilla activity and - while this makes solution of the refugee problem even more remote - emotionally Indians are encouraged. It is useless, they feel, for Pakistan to go on pretending all is well.

Military commentators here expect further escalation in partisan activity in the next two months - which, coupled with the famine imminent in East Bengal, could paralyse any attempts by the military regime to create a semblance of order. But if people here derive satisfaction from the thought of retribution falling on the Bengalis' oppressors, natural justice will prove cold comfort. Every day the situation worsens in East Bengal will bring an exodus to India. And nothing would be gained if Indo-Pakistan relations were once again to demand UN intervention to patch up their borders in the aftermath of what would be, tragically, a war open to the very misinterpretation Islamabad would welcome and New Delhi fears.