这里的were是否应该用was

However, a Red Cross effort to bring in aid and evacuate residents from the shelling in Matiupol were frustrated for a second day in a row.

这句话的主语是a... effort吧，后面是两个不定式作定语对吗？所以我觉得were应该用was才对。还请老师指教！

是的，主语为effort，谓语应该用单数was。外媒也难免会出错的。