While it is true that the old USSR mediated in 1965, that mediation failed and the hottest part of the 1965 India-Pakistan War ensued after, not prior to, the USSR mediation.
The US could hardly have mediated as we had (and I was the Liaison Officer at the US Embassy then in Karachi for) the 6937tj Communications Group in Badabur, suburban Peshawar, and were under a bilateral treaty with Pakistan for that base, which was more important to the US fighting the Cold War than a mediation effort could have been.
If you will read the lenghty article I wrote a few months back on the COLD WAR TIMES, an arm of the Smithsonian Museum, the Cold War Museum, as copied by me onto this PDF site, you can see what we (the US Embassy of both Karachi and Delhi, in a joint meeting held in Karachi) did do to slow down and logistically end the 1965 India-Pakistan War.
As regards CENTO and SEATO Pakistan hosted some social dos I attended as a USAF junior officer held in Karachi both ashore and on a US Navy in harbor war ship. But President Ayub Khan was blatantly trying to use both CENTO and SEATO (focused on the Vietnam War which went hot while I was stationed in then West Pakistan) as a lever...which failed....President Ayub wanted US intelligence (my outfit) used to aide him and FM Bhutto, and General Musa's scheme, to attack India, and we, the US, said flatly NO.
President Ayub's benefit from being a member of primarily CENTO (Pakistan never did anything to help out in SEATO) was a huge US Military Advisory Assitance Program in and to Paksitan, both West and East Paksitan, and tons of US AID programs and related money. Pakistan came out just fine while the allies, including the US, apart from the lease on the base in Peshawar area, sucked air!