US STOCKS-S&P, Dow weaker after Ukraine tension escalates
* Yellen says Fed should be cautious in rate hike decision
* Russia aid convoy crossed border without OK, Ukraine says
* Indexes: Dow off 0.05 pct, S&P off 0.13 pct, Nasdaq up 0.3 pct (Updates to afternoon)
By Akane Otani
NEW YORK, Aug 22 (Reuters) - The S&P and Dow were weaker on Friday as tensions in Ukraine escalated and investors got few clues about the course of interest rates from Federal Reserve Chair Janet Yellen.
Ukraine declared on Friday that Russia had launched a "direct invasion" of its territory after Moscow sent a convoy of trucks across the border into eastern Ukraine.
NATO Secretary General Anders Fogh Rasmussen condemned the move, describing the situation as "an alarming build-up" and calling on Russia to "stop destabilising Ukraine."
Yellen, in a speech at a central banking conference in Jackson Hole, Wyoming, said U.S. labor markets remain hampered by the effects of the Great Recession. Yellen said the Federal Reserve should move cautiously in determining when to raise interest rates.
"I think the tone from Yellen's remarks were more hawkish than what the market was anticipating, because data has been coming in better than expected, and she's trying to keep her options open with interest rates," said Karyn Cavanaugh, senior market strategist with Voya Investment Management. Continuación...