By Michelle Nichols NEW YORK, July 6 (Reuters) - Russia skipped U.N. Security Council negotiations on Tuesday on whether to extend approval for cross-border aid access into Syria, diplomats said, as China pushed for "solutions" to its concerns about unilateral sanctions and aid transparency. The council mandate for the long-running U.N. operation delivering aid into northern Syria from Turkey is due to expire on Saturday. Veto-powers Russia and China are wary of renewing it, while Western members want to re-authorize and expand it. U.N. officials say there can be no substitute for the cross-b...