Officers say there are not enough of them to cope with the large number of migrants crossing the area in the hope of reaching Britain.
The agreement announced on Saturday provides for tighter border controls and increased cooperation between French and British police.
British immigration minister says fencing could replace the 'inadequate' barriers at the French port as tensions rise over migrant issue.
Amid growing concerns over migrants in Calais, Lord Howard says France has 'lost control' of frontiers under Schengen Agreement.
Natacha Bouchart says town is held 'hostage' by about 1,300 migrants and will block port unless UK does more to control their numbers.
Police in used tear gas to stop a third recent incident of fighting between Sudanese and Eritrean migrants in the French channel port.
Officers clear makeshift camp housing some 500 migrants at a food distribution point close to the French port where they had been squatting.
The insalubrious camps of migrants hoping to reach Britain, were razed for what officials said were health and safety reasons.
Interior minister wants 'renegotiation' of the Anglo-French agreement which led to closure of notorious Sangatte refugee camp in 2002.
Dozens of Syrian refugees in France, who have blocked an entry ramp to English Channel ferries in Calais, say they are determined to get to UK.
This week a major national conference has been taking place to highlight the crippling level of poverty on France, which affects around 8.6 million people. Ministers, officials, trade unions representatives and workers from local voluntary associates have been meeting to thrash out a plan of action. Mediapart's Rachida El Azzouzi travelled to Calais in north-west France to see the reality of poverty at first hand. In one area of the town she found that all that is handed down from generation to generation is poverty and unemployment.