All air and sea ports have been put on alert for missing 14-year-old girl and 35-year-old John Bush.
Police say sightings suggest they are heading south from Yorkshire and that any move to leave the country would be "very foolish".
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The 100 leaders did not agree on concrete proposals, but the meeting has raised hopes for concrete international agreement next December.
The speech was OK, passionate at times and certainly well received, but did not display the passion of past speeches.
The brother-in-law of a British hostage being held by Islamic State militants in Syria, says he is scared by US airstrikes in the country.