I have a Pakistani Passport which has valid USA and UK multiple entry visas on it. Am I allowed to enter or transit through Serbia particularly Belgrade.
Yes, you can enter or transit through Belgrade airport or enter any Serbian Port of Entry without any visa.
From the official Ministry Of Foreign Affairs of the Republic Of Serbia website:
At its meeting of 30 October 2014, the Government of the Republic of Serbia adopted the Decision on visa free entry to the Republic of Serbia for holders of foreign passports having a valid Schengen, UK and other Member States' visa, or visa of the United States of America, and for holders of foreign passports having residence permit in the countries of the Schengen area, EU or the United States of America, which was published in the "Official Gazette of RS", No. 119 of 31 October 2014 and which will enter into force on 8 November 2014.
By this decision, the above-mentioned categories of foreign nationals may, without prior visa application, enter transit or stay in the Republic of Serbia up to 90 days during a six-month period, but not exceeding the expiring date of the said visas or residence permits.