Quote Originally Posted by maxmixiv View Post
Yes, or, in order to have some pity on the tongue, could be also "принимавшие участие".
"которые участвовали" is okay, too.

But should not be the English version 'who were involved', I wonder?

If negotiations are going on right now, then we have Present, and it changes everything:
участвующие
принимающие участие
которые участвуют
...
who are involved = которые участвуют (принимают участие) - present tense.
who were involved = которые участвовали (принимали участие) - past tense.

Also, in Russian we usually say "вести переговоры". So, the the best translation of your sentense is:
Послы, которые ведут переговоры, отказались делать какие-либо заявления.